c++: Fix implicit friend operator==.

c++: Fix implicit friend operator==.

It seems that in writing testcases for the operator<=> proposal I didn't
include any tests for implicitly declared friend operator==, and
consequently it didn't work.

2020-02-11  Jason Merrill  <jason@redhat.com>

	PR c++/93675
	* class.c (add_implicitly_declared_members): Use do_friend.
	* method.c (implicitly_declare_fn): Fix friend handling.
	(decl_remember_implicit_trigger_p): New.
	(synthesize_method): Use it.
	* decl2.c (mark_used): Use it.