[clang] [clang] Add `__has_feature(swiftcc)` support (PR #85347)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

You should also add documentation for this to https://github.com/llvm/llvm-project/blob/main/clang/docs/LanguageExtensions.rst and a release note for the change in https://github.com/llvm/llvm-project/blob/main/clang/docs/ReleaseNotes.rst.

https://github.com/llvm/llvm-project/pull/85347