[SVFS] Vector Function ABI demangling.

This patch implements the demangling functionality as described in the
Vector Function ABI. This patch will be used to implement the
SearchVectorFunctionSystem (SVFS) as described in the RFC:

http://lists.llvm.org/pipermail/llvm-dev/2019-June/133484.html

A fuzzer is added to test the demangling utility.

Patch by Sumedh Arani <sumedh.arani@arm.com>

Differential revision: https://reviews.llvm.org/D66024

llvm-svn: 372343