Proof of concept moving of generic directive parsing from AsmParser to the

Proof of concept moving of generic directive parsing from AsmParser to the
GenericAsmParser extension, where a lot of directives are already being parsed.
The end goal is having just a single place (and a single lookup table) for
all directive parsing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172268 91177308-0d34-0410-b5e6-96231b3b80d8