[clang] [clang] Lower modf builtin using `llvm.modf` intrinsic (PR #126750)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Can you just move the modf handling into the section for non-math intrinsics?  It's not a "math intrinsic" in this context: it doesn't set errno or raise exceptions.

https://github.com/llvm/llvm-project/pull/126750