[clang] [libcxx] [Clang] Allow __builtin_fma/fmaf/fmal to be used in a constant expression (PR #158048)

[Erich Keane via cfe-commits]

erichkeane wrote:

Looks like @hubert-reinterpretcast should make another pass at review/tell us when he thinks this is ready/good.

Additionally, CI is pretty grumpy here :)  Is any of that due to this patch?

https://github.com/llvm/llvm-project/pull/158048