[PATCH] Change .thumb_set to have the same error checks as .set

[Rafael Espíndola]

> Oh wait, i was wrong.
>
> So AsmParser.cpp doesn’t have a header file.  Its a subclass of MCAsmParser.
> ARMAsmParser itself is a parser extension, but (obviously given there’s no
> header), there’s no way to get from a parser extension to AsmParser.
>
> Everything is connected once you’re down at the MC level, but I didn’t want
> to expose this functionality in MCAsmParser as it doesn’t fit in with the
> other methods there.

I guess I would have a small preference for making
parseAssignmentExpression a method of MCAsmParser, but the current
patch LGTM.

Cheers,
Rafael