[clang] [llvm] [RFC][MATHVEC] Introduce the LLVM's mathvec library as yet another veclib functions provider (PR #234621)

[Paul Osmialowski via cfe-commits]

pawosm-arm wrote:

> I've marked the AdvSIMD vector routines with aarch64_vector_pcs, which has merged (#235009), so you can probably change `NOCC` to `CallingConv::AArch64_VectorCall`

Thanks @DylanFleming-arm, I've updated my PR accordingly.

https://github.com/llvm/llvm-project/pull/234621