Move ParseMacroArgument to the MCAsmParser interfance.

Since it's used by extensions. One further step to fully decoupling
GenericAsmParser from an intimate knowledge of the internals of AsmParser,
pointing it to the MCASmParser interface instead (like all other parser
extensions do).

Since this change moves the MacroArgument type to the interface header, it's
renamed to be a bit more descriptive in a general context.

llvm-svn: 172449