Specify Clang vector builtins.

This patch specifies a set of vector builtins for Clang, as discussed on
cfe-dev:
https://lists.llvm.org/pipermail/cfe-dev/2021-September/068999.html
https://lists.llvm.org/pipermail/cfe-dev/2021-October/069070.html

Reviewed By: scanon

Differential Revision: https://reviews.llvm.org/D111529