c++: Make spell corrections consistent

c++: Make spell corrections consistent

My change to namespace-scope spell corrections ignored the issue that
different targets might have different builtins, and therefore perturb
iteration order.  This fixes it by using an intermediate array of
identifier, which we sort before considering.

	gcc/cp/
	* name-lookup.c (maybe_add_fuzzy_decl): New.
	(maybe_add_fuzzy_binding): New.
	(consider_binding_level): Use intermediate sortable vector for
	namespace bindings.
	gcc/testsuite/
	* c-c++-common/spellcheck-reserved.c: Restore diagnostic.