[c++] Adjust mark used member in instantiated class scope

[c++] Adjust mark used member in instantiated class scope

The fix for PR97966 caused a regression with (non-template) member
functions of template classes.  We need to mark them used in the
instantiated class's scope, rather than the scope we were in before
instantiating, as the latter may itself be in template and change the
behaviour of marking a function as used.

	gcc/cp/
	* pt.c (instantiate_class_template_1): Process attribute((used)) set
	in class's context.
	gcc/testsuite/
	* g++.dg/template/attr-used.C: New.