c++: erroneous partial spec vs primary tmpl [PR116064]

When a partial specialization is deemed erroneous at parse time, we
currently flag the primary template as erroneous instead.  Later
at instantiation time we check if the primary template is erroneous
rather than the selected partial specialization, so at least we're
consistent.

But it's better not to conflate a partial specialization with the
primary template since they're instantiated independenty.  This avoids
rejecting the instantiation of A<int> in the below testcase.

	PR c++/116064

gcc/cp/ChangeLog:

	* error.cc (get_current_template): If the current scope is
	a partial specialization, return it instead of the primary
	template.
	* pt.cc (instantiate_class_template): Pass the partial
	specialization if any to maybe_diagnose_erroneous_template
	instead of the primary template.

gcc/testsuite/ChangeLog:

	* g++.dg/template/permissive-error2.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>