[clang] [Clang] Define `__WCHAR_MIN__` and related macros (PR #208135)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! The changes should also come with a release note in `clang/docs/ReleaseNotes.md`.

https://github.com/llvm/llvm-project/pull/208135