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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175587 91177308-0d34-0410-b5e6-96231b3b80d8