c++: modules and tsubst_friend_class

In 20_util/function_objects/mem_fn/constexpr.cc we start to instantiate
_Mem_fn_base's friend declaration of _Bind_check_arity before we've loaded
the namespace-scope declaration, so lookup_imported_hidden_friend doesn't
find it.  But then we load the namespace-scope declaration in
lookup_template_class during substitution, and so when we get around to
pushing the result of substitution, they conflict.  Fixed by calling
lazy_load_pendings in lookup_imported_hidden_friend.

gcc/cp/ChangeLog:

	* name-lookup.cc (lookup_imported_hidden_friend): Call
	lazy_load_pendings.