Encapsulate the MacroEnabled flag in AsmParser behind accessor methods.

The methods are also exposed via the MCAsmParser interface, which allows more
than one client to control them. Previously, GenericAsmParser was playing with
a member var in AsmParser directly (by virtue of being its friend).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172440 91177308-0d34-0410-b5e6-96231b3b80d8