c++: defaulted op== for incomplete class [PR107291]

After complaining about lack of friendship, we should not try to go on and
define the defaulted comparison operator anyway.

	PR c++/107291

gcc/cp/ChangeLog:

	* method.cc (early_check_defaulted_comparison): Fail if not friend.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-eq17.C: New test.