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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Referring to the C standard and implementations, I think modf does in fact raise an fp exception when the operand is an snan.  So to handle this, we need different lowering in strictfp mode, probably to a "constrained" intrinsic.

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