[clang] [clang] Add `__builtin_sincospi` that lowers to `llvm.sincospi.*` (PR #127065)

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

I've disabled the intrinsic lowering for both `sincos`/`sincospi` when strictfp is enabled (and added a test for that). 

I think I might need to cherry-pick the `sincos` fix onto the release 20.x branch too.

https://github.com/llvm/llvm-project/pull/127065