As it stands, the OperandMatchResultTy is only included in the generated
header if there is custom operand parsing. However, almost all backends
make use of MatchOperand_Success and friends from OperandMatchResultTy for
e.g. parseRegister. This is a pain when starting an AsmParser for a new
backend that doesn't yet have custom operand parsing. Move the enum to
MCTargetAsmParser.h.

This patch is a prerequisite for D23563

Differential Revision: https://reviews.llvm.org/D23496

llvm-svn: 285705