Reapply "[libc++][C++03] Copy the LLVM 19 headers (#108999)"

[philnik777]

This reverts commit 68c04b0.

This disables the IWYU mapping that caused the failure, since
the headers aren't reachable for now.

This is the first part of the "Freezing C++03 headers" proposal explained in https://discourse.llvm.org/t/rfc-freezing-c-03-headers-in-libc/77319/58.

This patch mechanically copies the headers as of the LLVM 19.1 release into a subdirectory of libc++ so that we can start using these headers when building in C++03 mode. We are going to be backporting important changes to that copy of the headers until the LLVM 21 release. After the LLVM 21 release, only critical bugfixes will be fixed in the C++03 copy of the headers.

This patch only performs a copy of the headers -- these headers are still unused by the rest of the codebase.