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

[via cfe-commits]

dcandler wrote:

FYI, I was also a getting an error from this change, when trying to build picolibc for armv6m (as well as other arm targets):
```
SoftenFloatResult #0: t106: f64,f64 = fmodf t105, ../../../../../src/picolibc/test/math-funcs.c:85:10
fatal error: error in backend: Do not know how to soften the result of this operator!
```
The error is gone now the change has been reverted, but it's slightly different to the one reported above.

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