When querying type qualifiers on QualType via one of the "non-local"

interfaces (which are used throughout the front end), combine the
qualifiers on the QualType instance with the qualifiers on the
canonical type to produce the set of qualifiers that, semantically,
apply to that type. This should design away a large category of
"qualifier-hidden-behind-a-typedef" buts like we saw in PR5383.

Performance-wise, this caused a regression of ~0.5% on Cocoa.h, but
it's totally worth it. We may actually be able to get a little more
performance back by using CanQualType more often.

llvm-svn: 89018