Fix our handling of visibility in explicit template instantiations.

* Don't copy the visibility attribute during instantiations. We have to be able
  to distinguish

 struct HIDDEN foo {};
 template<class T>
 DEFAULT void bar() {}
 template DEFAULT void bar<foo>();

from

 struct HIDDEN foo {};
 template<class T>
 DEFAULT void bar() {}
 template void bar<foo>();

* If an instantiation has an attribute, it takes precedence over an attribute
  in the template.

* With instantiation attributes handled with the above logic, we can now
  select the minimum visibility when looking at template arguments.

llvm-svn: 156821