[clang] [clang-cl] Fix value of __FUNCTION__ in MSVC mode. (PR #84014)

[via cfe-commits]

Sirraide wrote:

I’m not entirely sure this change is a good idea: some people might be relying on Clang’s behaviour wrt `__FUNCTION__`, especially with how long it’s been around; Clang may not be fully MSVC-compatible in this case, but that incompatibilty seems unlikely to cause problems imo since afaik most people that actually depend on the output of these sorts of macros for e.g. reflexion use `__FUNCSIG__` or `__PRETTY_FUNCTION__` instead—then again, that could also be a reason as to why changing it might actually be fine.

https://github.com/llvm/llvm-project/pull/84014