[TableGen] Move OperandMatchResultTy enum to MCTargetAsmParser.h

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