[Cling] Fail to detect type as template if declared by using in a namespace

The following reproducer gives true for CXX17 but false for CXX11 and CXX14. This causes follow up bugs in PyROOT so that eventually we cannot resolve ROOT::RVec, which is a using declaration pulled in from ROOT::VecOps::RVec.

template <class T> class Foo { T x; };

namespace Bar {

    using ::Foo;

}

void ns_lookup() {

    cout << gInterpreter->CheckClassTemplate("Bar::Foo") << endl;

}

The mechanic is called by experimental PyROOT/cppyy to detect a template (see Cppyy::IsTemplate). This is a blocker for 6.22.