[clang] [clang] constexpr built-in fma function. (PR #113020)

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c8ef wrote:

Actually, my reason for this patch is not necessarily related to libc++, but more about #51787. This issue requires the implementation of constexpr vector builtins. After some attempts, I found that some scalar versions of them cannot be used in a constexpr context. Therefore, I decided to implement those first.

Regarding the patch itself, is it good to go or does it need further revision? @philnik777 

https://github.com/llvm/llvm-project/pull/113020