c++: variable partial spec redeclaration [PR113612]

If register_specialization finds a previous declaration and throws the new
one away, we shouldn't still add the new one to
DECL_TEMPLATE_SPECIALIZATIONS.

	PR c++/113612

gcc/cp/ChangeLog:

	* pt.cc (process_partial_specialization): Return early
	on redeclaration.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ85.C: New test.