c++: less invalidate_class_lookup_cache

c++: less invalidate_class_lookup_cache

In the testcase below, we push_to_top_level to instantiate f and g, and they
can both use the previous_class_level cache from instantiating A<int>.
Wiping the cache in pop_from_top_level is not helpful; we'll do that in
pushclass if needed.

  template <class T> struct A
  {
    int i;
    void f() { i = 42; }
    void g() { i = 24; }
  };

  int main()
  {
    A<int> a;
    a.f();
    a.g();
  }

gcc/cp/ChangeLog:

	* name-lookup.cc (pop_from_top_level): Don't
	invalidate_class_lookup_cache.