Make Sema::getTypeName return the opaque pointer of a QualType rather

than a Decl, which gives us some more flexibility to express the
results with the type system. There are no clients using this
flexibility yet, but it's meant to be able to describe qualified names
as written in the source (e.g., "foo::type") or template-ids that name
a class template specialization (e.g., "std::vector<INT>").

DeclSpec's TST_typedef has become TST_typename, to reflect its use to
describe types found by name (that may or may not be typedefs). The
type representation of a DeclSpec with TST_typename is an opaque
QualType pointer. All users of TST_typedef, both direct and indirect,
have been updated for these changes.

llvm-svn: 64141