[AArch64] Add parsing of aarch64_vector_pcs attribute.

This patch adds parsing support for the 'aarch64_vector_pcs'
calling convention attribute to calls and function declarations.

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

Reviewers: t.p.northover, rnk, rengolin, javed.absar, thegameg, SjoerdMeijer

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D51477

llvm-svn: 342030