c++: Template hash access

This exposes the template specialization table, so the modules
machinery may access it.  The hashed entity (tmpl, args & spec) is
available, along with a hash table walker.  We also need a way of
finding or inserting entries, along with some bookkeeping fns to deal
with the instantiation and (partial) specialization lists.

	gcc/cp/
	* cp-tree.h (struct spec_entry): Moved from pt.c.
	(walk_specializations, match_mergeable_specialization)
	(get_mergeable_specialization_flags)
	(add_mergeable_specialization): Declare.
	* pt.c (struct spec_entry): Moved to cp-tree.h.
	(walk_specializations, match_mergeable_specialization)
	(get_mergeable_specialization_flags)
	(add_mergeable_specialization): New.