[llvm] [AArch64][VecLib] Add vector function mappings for the modf, sincos, sincospi vector intrinsics (PR #175098)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue approved this pull request.

LGTM 

I find the sincos/sincospi names a little strange, but as far as I can tell they're correct (looking at `libamath`).

https://github.com/llvm/llvm-project/pull/175098