c++: C++17/20 class layout divergence [PR120012]

c++: C++17/20 class layout divergence [PR120012]

C++20 made a class with only explicitly defaulted constructors no longer
aggregate, and this wrongly affected whether the class is considered "POD
for layout purposes" under the ABI.

Conveniently, we already have check_non_pod_aggregate to diagnose cases
where this makes a difference, due to PR103681 around a C++14 aggregate
change.

	PR c++/120012

gcc/cp/ChangeLog:

	* cp-tree.h (struct lang_type): Add non_aggregate_pod.
	(CLASSTYPE_NON_AGGREGATE_POD): New.
	* class.cc (check_bases_and_members): Set it.
	(check_non_pod_aggregate): Diagnose it.

gcc/ChangeLog:

	* doc/invoke.texi: Document C++20 aggregate fix.
	* common.opt: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/base-defaulted1.C: New test.
	* g++.dg/abi/base-defaulted1a.C: New test.