[clang] [clang-cl] Fix value of __FUNCTION__ and __FUNC__ in MSVC mode for c++. (PR #66120)

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> This implementation has bugs, see [#66114 (comment)](https://github.com/llvm/llvm-project/issues/66114#issuecomment-1732319259)

@RIscRIpt Thanks. Will look at them.

https://github.com/llvm/llvm-project/pull/66120