[clang] `__builtin_FUNCSIG()` intrinsic.

[fsb4000]

Microsoft C++ supports __builtin_FUNCSIG() since the latest preview ( VS 2022 17.5 Preview 1).
It is similar to __builtin_FUNCTION() but returns the same string as __FUNCSIG__.
It would be great if clang-cl supports the intrinsic too.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[JMazurkiewicz]

I've opened https://reviews.llvm.org/D150183.