c++: redundant hashing in register_specialization

After r15-4050-g5dad738c1dd164 register_specialization needs to set
elt.hash to the (maybe) precomputed hash so that the lookup uses it
rather than redundantly computing it from scratch.

gcc/cp/ChangeLog:

	* pt.cc (register_specialization): Set elt.hash.

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