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

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

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

With the current `modf` builtin (not changed here) it skips the code path for emitting intrinsics when `math-errno` is enabled. I think setting `ConstIgnoringExceptions` on the builtin would allow the intrinsics even with `math-errno` (though that's not necessary for our use case). 

https://github.com/llvm/llvm-project/blob/0121d99af4d121242becc05941c939cad84105d4/clang/lib/CodeGen/CGBuiltin.cpp#L3024-L3035

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