[clang] [clang] Add sincos builtin using `llvm.sincos` intrinsic (PR #114086)

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

> CC @rohitaggarwal007 who added sincos vectorisation for amdlibm recently - hopefully we can get ensure amdlibm uses the new builtin + intrinsic safely

I have another patch #114039, that allows lowering the `llvm.sincos` intrinsic to the existing vector function mappings. So once the vectorizer can handle this intrinsic, it should work :slightly_smiling_face:  

https://github.com/llvm/llvm-project/pull/114086