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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> when -fno-math-errno is set 

What does -fno-math-errno have to do with anything?  modf never sets errno anyway.

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