Fix a point of semantics with using declaration hiding: method templates

introduced by using decls are hidden even if their template parameter lists
or return types differ from the "overriding" declaration.

Propagate using shadow declarations around more effectively when looking up
template-ids.  Reperform lookup for template-ids in member expressions so that
access control is properly set up.

Fix some number of latent bugs involving template-ids with totally invalid
base types.  You can only actually get these with a scope specifier, since
otherwise the template-id won't parse as a template-id.

Fixes PR7384.

llvm-svn: 106093