c++: ICE with using-decl [PR 99238]

c++: ICE with using-decl [PR 99238]

This ICE was caused by a stray TREE_VISITED marker.  The lookup
machinery was leaving it there due to the way I'd arranged for it to
be cleared.  That was presuming the name_lookup::value field didn't
change, and that wasn't always true in the using-decl processing.  I
took the opportunity to break out a helper, and then call it
immediately after lookups, rather than wait until destructor time.
Added some asserts the module machinery to catch further cases of
this.

	PR c++/99238
	gcc/cp/
	* module.cc (depset::hash::add_binding_entity): Assert not
	visited.
	(depset::add::add_specializations): Likewise.
	* name-lookup.c (name_lookup::dedup): New.
	(name_lookup::~name_lookup): Assert not deduping.
	(name_lookup::restore_state): Likewise.
	(name_lookup::add_overload): Replace outlined code with dedup
	call.
	(name_lookup::add_value): Likewise.
	(name_lookup::search_namespace_only): Likewise.
	(name_lookup::adl_namespace_fns): Likewise.
	(name_lookup::adl_class_fns): Likewise.
	(name_lookup::search_adl): Likewise.  Add clearing dedup call.
	(name_lookup::search_qualified): Likewise.
	(name_lookup::search_unqualified): Likewise.
	gcc/testsuite/
	* g++.dg/modules/pr99238.h: New.
	* g++.dg/modules/pr99238_a.H: New.
	* g++.dg/modules/pr99238_b.H: New.