c++: op== defaulted outside class [PR110084]

defaulted_late_check is for checks that need to happen after the class is
complete; we shouldn't call it sooner.

	PR c++/110084

gcc/cp/ChangeLog:

	* pt.cc (tsubst_function_decl): Only check a function defaulted
	outside the class if the class is complete.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-synth-neg3.C: Check error message.
	* g++.dg/cpp2a/spaceship-eq16.C: New test.