Forbid qualifiers on ObjC generic parameters and arguments, but

silently ignore them on arguments when they're provided indirectly
(.e.g behind a template argument or typedef).

This is mostly just good language design --- specifying that a
generic argument is __weak doesn't actually do anything --- but
it also prevents assertions when trying to apply a different
ownership qualifier.

rdar://21612439

llvm-svn: 248436