Implement DR 757...

Implement DR 757...

	Implement DR 757: It's OK for a decl to use a type without linkage
	so long as the decl is defined in the current translation unit.
	* decl2.c (no_linkage_decls): New vector.
	(mark_used): Add decls that use types with no linkage.
	(cp_write_global_declarations): Check that they are defined.
	(decl_defined_p, no_linkage_error): New fns.
	* cp-tree.h (DECL_NO_LINKAGE_CHECKED): New macro.
	(struct lang_decl_base): Add flag.
	* decl.c (grokfndecl): Don't check type linkage.
	(grokvardecl): If the type has no linkage, just make sure
	DECL_LANG_SPECIFIC is set.
	* pt.c (check_instantiated_arg): Don't check type linkage.
	* name-lookup.c (is_local_extern): New fn.
	* name-lookup.h: Declare it.

From-SVN: r150634