[AArch64] Implement Vector Funtion ABI name mangling.

Summary:
The name mangling scheme is defined in section 3.5 of the "Vector function application binary interface specification for AArch64" [1].

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

Reviewers: rengolin, ABataev

Reviewed By: ABataev

Subscribers: sdesmalen, javed.absar, kristof.beyls, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60583

llvm-svn: 358490