Adjust duplicate checker

Adjust duplicate checker

	gcc/cp/
	* cp-tree.h (comparing_typenames): Replace with ...
	(map_context_from, map_context_to): ... this.
	* typeck.c (structural_comptypes): Adjust.
	* pt.c (comparing_typenames): Delete.
	(spec_hasher::equal): Adjust.
	* tree.c (cp_tree_equal): Check map_context_from & to for failed
	parameter context.
	* module.cc (map_context_from, map_context_to): Define.
	(check_mergeable_decl): Set & unset map_context vars.
	(trees_in::is_matching_decl): Likewise.
	(module_state::read_cluster): Set & unset
	comparing_specializations, not comparing_typenames.