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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> Oh, I just noticed that you’ve tried to land this change before (#66120), but it seems it was reverted because of some problems; if those are fixed now, then I’d say going forward with this is probably fine, seeing as it was approved the first time round.
> 
> This would have been a bit less confusing if you had pointed that out, just saying ;Þ

Yes. Sorry about that. This patch was mostly to fix the issues raised in PR#66120.

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