Document supported libstdc++ versions

[cor3ntin]

Clang carries a number of hacks for older libstdc++ versions, some for libstdc++ 4.4 (which can be found by searching for "libstdc++" in the clang subtree).

It would be useful to document the minimal version of libstdc++ we think Clang should be compatible with (which would hopefully be tested by CI), and from there, clean up any such workarounds we no longer need.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: cor3ntin (cor3ntin)

Clang carries a number of hacks for older libstdc++ versions, some for libstdc++ 4.4.

It would be useful to document the minimal version of libstdc++ we think Clang should be compatible with (which would hopefully be tested by CI), and from there, clean up any such workarounds we no longer need.

[philnik777]

You might also want to think about a minimum supported libc++ version. IIRC there were some workarounds added for some ancient libc++ version in the std::move builtin family. (Personally I don't think something like the last 4 releases would be unreasonable, but that's not my call to make)

[efriedma-quic]

6ad7e87 set the minimum at 4.8, I think?