c++: bad ggc_free in try_class_unification [PR109556]

Aside from correcting how try_class_unification copies multi-dimensional
'targs', r13-377-g3e948d645bc908 also made it ggc_free this copy as an
optimization.  But this is wrong since the call to unify within might've
captured the args in persistent memory such as the satisfaction cache
(as part of constrained auto deduction).

	PR c++/109556

gcc/cp/ChangeLog:

	* pt.cc (try_class_unification): Don't ggc_free the copy of
	'targs'.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-placeholder13.C: New test.

(cherry picked from commit 5e284ebb)