[clang] Reland "[clang] Lower modf builtin using `llvm.modf` intrinsic" (PR #129885)

[Nico Weber via cfe-commits]

nico wrote:

Here are a few ideas for possible paths forward: https://issues.chromium.org/issues/401571943#comment9

…actually, let me inline them:

`if (Builder.getIsFPConstrained())` -> `if (Builder.getIsFPConstrained() && !isTargetingWin32())`?

Alternatively, we could put an actual symbol in compiler-rt that calls modff from the CRT, which puts an available symbol into compiler-rt. (…can compiler-rt depend on CRT?)

There's probably nothing that guarantees that the symbol isn't inline on other platforms either. …but we do have the inline symbol. Maybe LLVM doesn't know to mark the inline as used when it's called through the intrinsic? That could be a third idea to look into.

https://github.com/llvm/llvm-project/pull/129885