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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic requested changes to this pull request.

Hang on... I need to think a little more about the interaction with snan in strictfp mode.

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