[clang] [Clang] Add __builtin_bswapg (PR #162433)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt commented:

I'd think this is something that @AaronBallman or similar should approve (spelling at least).

There should be actual documentation about this in LanguageExtensions.rst (it seems like like the existing ones don't have documentation, but maybe they should be placed into a group just documenting the family of builtins), and a note in ReleaseNotes.rst

https://github.com/llvm/llvm-project/pull/162433