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

[Chaitanya Koparkar via cfe-commits]

https://github.com/ckoparkar commented:

I still need to write tests. Trying to figure out the tablegen thing mentioned inline.

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