Don't cache the visibility of types.

Since r175326 an implicitly hidden template argument can cause a template
installation to become hidden, even if the template itself has an explicit
default visibility. This requires that we keep track of "late" additions
of the visibility attribute.

This is hopefully the last followup change. It just removes the caching of
visibilities from types so that we can see new attributes even after a type has
been used.

llvm-svn: 176164