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

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

Thanks for the report :+1: It looks like all we have to do is set the legalization rule for f32 to "Promote". Looks like there's actually already logic to do this for a bunch of other nodes:

>From X86ISelLowering.cpp:
```c++
  // On 32 bit MSVC, `fmodf(f32)` is not defined - only `fmod(f64)`
  // is. We should promote the value to 64-bits to solve this.
  // This is what the CRT headers do - `fmodf` is an inline header
  // function casting to f64 and calling `fmod`.
  if (Subtarget.is32Bit() &&
      (Subtarget.isTargetWindowsMSVC() || Subtarget.isTargetWindowsItanium()))
    // clang-format off
   for (ISD::NodeType Op :
         {ISD::FACOS,  ISD::STRICT_FACOS,
          ISD::FASIN,  ISD::STRICT_FASIN,
          ISD::FATAN,  ISD::STRICT_FATAN,
          ISD::FATAN2, ISD::STRICT_FATAN2,
          ISD::FCEIL,  ISD::STRICT_FCEIL,
          ISD::FCOS,   ISD::STRICT_FCOS,
          ISD::FCOSH,  ISD::STRICT_FCOSH,
          ISD::FEXP,   ISD::STRICT_FEXP,
          ISD::FFLOOR, ISD::STRICT_FFLOOR,
          ISD::FREM,   ISD::STRICT_FREM,
          ISD::FLOG,   ISD::STRICT_FLOG,
          ISD::FLOG10, ISD::STRICT_FLOG10,
          ISD::FPOW,   ISD::STRICT_FPOW,
          ISD::FSIN,   ISD::STRICT_FSIN,
          ISD::FSINH,  ISD::STRICT_FSINH,
          ISD::FTAN,   ISD::STRICT_FTAN,
          ISD::FTANH,  ISD::STRICT_FTANH})
      if (isOperationExpand(Op, MVT::f32))
        setOperationAction(Op, MVT::f32, Promote);
  // clang-format on
```

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