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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I suspect fma is not one of the functions we want to try to share, at least not short-term.  It's one of the core IEEE operations, LLVM has APFloat::fusedMultiplyAdd since forever, and there's probably some weirdness with dependencies if we try to share it because other operations use it.

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