Reland "[clang] Lower modf builtin using `llvm.modf` intrinsic" (#129885)
Reverts llvm/llvm-project#127987
Original description:
This updates the existing modf[f|l] builtin to be lowered via the
llvm.modf.* intrinsic (rather than directly to a library call).
The legalization issues exposed by the original PR (#126750) should have
been fixed in #128055 and #129264.