Before, the code was like
https://github.com/emscripten-core/emscripten/blob/dc1abd514b1bade135a01a4453a9ff6def0793b6/system/lib/libcxx/include/__locale_dir/locale_base_api.h#L12-L30

But now they are divided into two parts, one using the new API and the
other using old. See "Locale API reimplementation" above.
https://github.com/llvm/llvm-project/blob/ec28b8f9cc7f2ac187d8a617a6d08d5e56f9120e/libcxx/include/__locale_dir/locale_base_api.h#L116-L138

This adds Emscripten in the beginning of the "old" API list. (This has
to be the beginning; see emscripten-core#23414)

Before, the code was like
https://github.com/emscripten-core/emscripten/blob/dc1abd514b1bade135a01a4453a9ff6def0793b6/system/lib/libcxx/include/__locale_dir/locale_base_api.h#L12-L30

But now they are divided into two parts, one using the new API and the
other using old. See "Locale API reimplementation" above.
https://github.com/llvm/llvm-project/blob/ec28b8f9cc7f2ac187d8a617a6d08d5e56f9120e/libcxx/include/__locale_dir/locale_base_api.h#L116-L138

This adds Emscripten in the beginning of the "old" API list. (This has
to be the beginning; see emscripten-core#23414)

This updates libcxx and libcxxabi to LLVM 20.1.4:
https://github.com/llvm/llvm-project/releases/tag/llvmorg-20.1.4

Before going into each additional change of the PR, these are some
noteworthy changes from this LLVM release:

- Freezing C++03 headers
- RFC:
https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc/77319
  - PRs:
    - llvm/llvm-project#108999
    - llvm/llvm-project#109000
    - llvm/llvm-project#109001
    - llvm/llvm-project#109002
    - 
This copies libc++ headers of the last LLVM 19 release into a separate
directory
(https://github.com/llvm/llvm-project/tree/main/libcxx/include/__cxx03)
and redirects all C++03 workflow there. The motivation is not to fix
C++03 related changes unless they are critical bugs, and simplifies the
main headers. So the main headers are like
  ```
  #if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
  #  include <__cxx03/algorithm>
  #else
  ... main header content ...
  ```

As you can see it looks like we can avoid opting into this by not
defining `_LIBCPP_USE_FROZEN_CXX03_HEADERS` at the moment. I think their
eventual goal is to remove C++03 support from the main headers but it
hasn't seem to have happened yet and I don't know what their timeline
for that is.

Adding that `__cxx03` directory increases the header size by 10MB. We
can get away not using them by not defining
`_LIBCPP_USE_FROZEN_CXX03_HEADERS` in this release, so this update does
not include that directory.

- Sharing of headers between libc++ and libc (Project Hand in Hand)
- RFC:
https://discourse.llvm.org/t/rfc-project-hand-in-hand-llvm-libc-libc-code-sharing/77701
- PR: llvm/llvm-project#91651 (More are likely
to come)

This tries to share some libc headers from libcxx. libcxx's source files
can depend on headers from `libc/shared`, `libc/src/__support`,
`libc/include/llvm-libc-macros`, and `libc/include/llvm-libc-types`. And
it turns out libcxx can also depend on `libc/hdr`, even though the
directory is not in the diagram in the RFC.

These headers can be only included from `libcxx/src` and not
`libcxx/include`, to prevent libcxx's API from changing. So these
headers don't need to be copied into `cache/`.

- Locale API reimplementation
  It doesn't seem to have an RFC, but the PRs are:
  - llvm/llvm-project#113737
  - llvm/llvm-project#115176
  - llvm/llvm-project#115752
  - llvm/llvm-project#122489

It looks this aims provide a new way to define locale API for each
platform. Currently Apple, FreeBSD, MSVCRT, and Fuschia are using the
new API and the others are still using the old one:
https://github.com/llvm/llvm-project/blob/ec28b8f9cc7f2ac187d8a617a6d08d5e56f9120e/libcxx/include/__locale_dir/locale_base_api.h#L116-L138
For our purpose, adding `if defined(__EMSCRIPTEN__)` entry to the list
of "old" list seems to work for the moment, but we may need to move to
the new way eventually later.

---

Additional changes:

- Copy `vendor/llvm/default_assertion_handler.in` to
`__assertion_handler`:
aa53648
Our previous `__assertion_handler` was copied from
`libcxx/vendor/llvm/default_assertion_handler.in`. This updates our
`__assertion_handler` to the new
`libcxx/vendor/llvm/default_assertion_handler.in`. For what this file
is, see the PR description of #22994, with which the file was added.

- Remove `libcxx/include/__cxx03` directory:
d646f6b
As I described in "Freezing C++ headers" above, C++03 headers were
copied to `include/cxx03` to be "frozen". But by not defining
`_LIBCPP_USE_FROZEN_CXX03_HEADERS` we can still use the main headers.
This new `__cxx03` header directory is almost 10M and we are not using
it in this update, this deletes it.

- Define more variables in `__config_site`:
9912236
libcxx decides to almost all configuration macros to be defined. So
before it tested whether a macro was defined/undefined, and now it
assumes it is defined and tests whether its value is 1/0.

Before llvm/llvm-project#112094
`_config_site.in` used to use `#cmakedefine`, which only defined
variables when they were enabled, but now it uses `#cmakedefine01`,
which always defines variables and assigns 1 or 0 depending on whether
the feature is enabled.

So this change adds `#define` to each variable that `_config_site.in`
has an entry of. The value assigned is 1 if it was defined in our
previous Emscripten environment and 0 if not.

- Fix Emscripten's locale:
2ae01b0
Before, the code was like
https://github.com/emscripten-core/emscripten/blob/dc1abd514b1bade135a01a4453a9ff6def0793b6/system/lib/libcxx/include/__locale_dir/locale_base_api.h#L12-L30
But now they are divided into two parts, one using the new API and the
other using old. See "Locale API reimplementation" above.

https://github.com/llvm/llvm-project/blob/ec28b8f9cc7f2ac187d8a617a6d08d5e56f9120e/libcxx/include/__locale_dir/locale_base_api.h#L116-L138
This adds Emscripten in the beginning of the "old" API list. (This has
to be the beginning; see #23414)

- Import libc headers used by libcxx:
12a4ee4,
12a4ee4
and
43c8ce4
This imports a part of libc headers into `system/lib/llvm-libc`. The
imported directories are:
  - `libc/shared`
  - `libc/src/__support`
  - `libc/include/llvm-libc-macros`
  - `libc/include/llvm-libc-types`
  - `libc/hdr`
  See "sharing of headers between libc+ and libc" above for details.
This also applies llvm/llvm-project#133999,
which is a bugfix that has not be backported, which fixes the bug of
including from a wrong directory.

- `std::uncaught_exception` -> `std::uncaught_exceptions`:
1bf4e78
`std::uncaught_exception` has been deprecated in C++17, so it generates
a warning (which we treat as an error). Replaced it with
`std::uncaught_exceptions`, which returns the number of uncaught
exceptions (but can still be used as a boolean in the test).

- Remove `ryu_constants.h` and `ryu_long_double_constants.h` from libc:
5767ac4
These are not used from libcxx and `ryu_long_double_constants.h` is huge
(12M).