c++: template and clone fns for modules

c++: template and clone fns for modules

We need to expose build_cdtor_clones, it fortunately has the desired
API -- gosh, how did that happen? :) The template machinery will need
to cache path-of-instantiation information, so add two more fields to
the tinst_level struct.  I also had to adjust the
match_mergeable_specialization API since adding it, so including that
change too.

	gcc/cp/
	* cp-tree.h (struct tinst_level): Add path & visible fields.
	(build_cdtor_clones): Declare.
	(match_mergeable_specialization): Use a spec_entry, add insert parm.
	* class.c (build_cdtor_clones): Externalize.
	* pt.c (push_tinst_level_loc): Clear new fields.
	(match_mergeable_specialization): Adjust API.