c++: optimize constraint subsumption [PR118069]

c++: optimize constraint subsumption [PR118069]

Since atomic constraints are interned the subsumption machinery can
safely use pointer instead of structural hashing for them.  This speeds
up compilation of the testcase in the PR from ~3s to ~2s.

	PR c++/118069

gcc/cp/ChangeLog:

	* constraint.cc (atom_hasher): Define here, instead of ...
	* cp-tree.h (atom_hasher): ... here.
	* logic.cc (clause::m_set): Use pointer instead of structural
	hashing.

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