Do not implicitly instantiate the definition of a class template specialization

that has been explicitly specialized!

We assume in various places that we can tell the template specialization kind
of a class type by looking at the declaration produced by TagType::getDecl.
That was previously not quite true: for an explicit specialization, we could
have first seen a template-id denoting the specialization (with a use that does
not trigger an implicit instantiation of the defintiion) and then seen the
first explicit specialization declaration. TagType::getDecl would previously
return an arbitrary declaration when called on a not-yet-defined class; it
now consistently returns the most recent declaration in that case.

llvm-svn: 295118