Add a new 'type_visibility' attribute to allow users to

control the visibility of a type for the purposes of RTTI
and template argument restrictions independently of how
visibility propagates to its non-type member declarations.

Also fix r175326 to not ignore template argument visibility
on a template explicit instantiation when a member has
an explicit attribute but the instantiation does not.

The type_visibility work is rdar://11880378

llvm-svn: 175587