[llvm-mc/AsmParser] Using validateTargetOperandClass in llvm-mc

This is per Vladimir Medic:

Hi all,

I have looked at the commits 174488 and174487 by Jim Grosbach as this
is something that could be very useful for the mips llvm assembler that
I’m working on. In fact, I came up with a similar idea myself a few months
ago but I rejected it in the end for one reason only:

The MatchKindClass enumerated value is defined inside the
GET_MATCHER_IMPLEMENTATION definition which to my mind is a part
of the internal matcher implementation that is not safe to use outside
generated code.

We’ve seen this happened for the disassembler who’s implementation has
completely changed without affecting the C++ code. So how safe is it to use these
values from target AsmParser? If this is something that will stay in a code I’d
love to use it as it would make our implementation much easier.

Kind regards
Vladimir Medic