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

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Sirraide wrote:

Oh, I just noticed that you’ve tried to land this change before (https://github.com/llvm/llvm-project/pull/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

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