ParseStatus is slightly more convenient to use due to implicit
conversion from bool, which allows to do something like:
```
return Error(L, "msg");
```
when with MatchOperandResultTy it had to be:
```
Error(L, "msg");
return MatchOperand_ParseFail;
```
It also has more appropriate name since parse* methods are not only for
parsing operands.
Reviewed By: uweigand
Differential Revision: https://reviews.llvm.org/D154316
aeeaadd6ee