[AArch64] Add aarch64_vector_pcs function attribute to Clang

[AArch64] Add aarch64_vector_pcs function attribute to Clang

This is the Clang patch to complement the following LLVM patches:
  https://reviews.llvm.org/D51477
  https://reviews.llvm.org/D51479

More information describing the vector ABI and procedure call standard
can be found here:

https://developer.arm.com/products/software-development-tools/\
                          hpc/arm-compiler-for-hpc/vector-function-abi

Patch by Kerry McLaughlin.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D54425

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@347571 91177308-0d34-0410-b5e6-96231b3b80d8