Sema would directly check for the presence of the @objcMembers attribute,
and inherit it from the immediate superclass in validateDecl().
We don't want validateDecl() to have side effects like this and this was
already a problem, because it would not inherit the attribute transitively
in some cases.
Instead, add a ClassDecl::hasObjCMembers() method that walks over all
ancestors and caches the result.
<rdar://problem/46420252>