Do not implicitly instantiate the definition of a class template specialization

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@295118 91177308-0d34-0410-b5e6-96231b3b80d8