[AArch64] Implement Vector Funtion ABI name mangling.
The name mangling scheme is defined in section 3.5 of the "Vector function application binary interface specification for AArch64" .
Reviewers: rengolin, ABataev
Reviewed By: ABataev
Subscribers: sdesmalen, javed.absar, kristof.beyls, jdoerfert, llvm-commits
Differential Revision: https://reviews.llvm.org/D60583