c++: pushdecl_top_level must set context

c++: pushdecl_top_level must set context

I discovered pushdecl_top_level was not setting the decl's context,
and we ended up with namespace-scope decls with NULL context.  That
broke modules.  Then I discovered a couple of places where we set the
context to a FUNCTION_DECL, which is also wrong.  AFAICT the literals
in question belong in global scope, as they're comdatable entities.
But create_temporary would use current_scope for the context before we
pushed it into namespace scope.

This patch asserts the context is NULL and then sets it to the frobbed
global_namespace.

	gcc/cp/
	* name-lookup.c (pushdecl_top_level): Assert incoming context is
	null, add global_namespace context.
	(pushdecl_top_level_and_finish): Likewise.
	* pt.c (get_template_parm_object): Clear decl context before
	pushing.
	* semantics.c (finish_compound_literal): Likewise.