* cp-tree.h (CP_DECL_CONTEXT): New macro.
	* decl2.c (is_namespace_ancestor, lookup_using_namespace): Use it.
	* method.c (build_overload_nested_name): Likewise.
	* sig.c (build_signature_pointer_or_reference_type): Don't set
	DECL_CONTEXT.
	Set DECL_CONTEXT for globals to NULL_TREE instead of global_namespace.
	* cp-tree.h (FROB_CONTEXT): New macro.
	(DECL_MAIN_P): ::main should have a DECL_CONTEXT of NULL_TREE.
	* decl.c (namespace_binding): Replace NULL_TREE with
	global_namespace.
	(set_namespace_binding, pop_namespace, lookup_name_real): Likewise.
	* decl2.c (is_namespace_ancestor, lookup_using_namespace):
	Likewise.
	* decl.c (pushtag): Use FROB_CONTEXT.
	(pushdecl, make_typename_type, define_function, grokdeclarator):
	Likewise.
	* decl2.c (set_decl_namespace, do_namespace_alias): Likewise.
	* pt.c (push_template_decl_real, lookup_template_class, tsubst):
	Likewise.
	* decl2.c (decl_namespace): Return global_namespace if no context.
	* method.c (build_overload_nested_name): Expect null as context.
	* pt.c (mangle_class_name_for_template): Do nothing for null
	contexts.
	(lookup_template_class): Allow for null id_context.

From-SVN: r20710