[clang-tools-extra] [clang-cl] Fix value of __FUNCTION__ in MSVC mode. (PR #67592)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> Could you add a description explaining what is currently wrong with the value of `__FUNCTION__` is MSVC mode? I think we also need a release note.

Hope the RN is at the right place? Thanks.

https://github.com/llvm/llvm-project/pull/67592