[MC] Shrink MCAsmParser by grouping bools, add const, NFC

I was considering adding another boolean here. I standardized on bools
since they allow default member initializers in the class definition.
This makes ShowParsedOperands protected instead of private, but that's
probably fine.

Reduce the SmallVector size while we're at it, since the common case is
that there is never a pending error.

llvm-svn: 344967