c++: layout of aggregate base with DMI [PR103681]

c++: layout of aggregate base with DMI [PR103681]

C++14 changed the definition of 'aggregate' to allow default member
initializers, but such classes still need to be considered "non-POD for the
purpose of layout" for ABI compatibility with C++11 code.  It seems rare to
derive from such a class, as evidenced by how long this bug has
survived (since r216750 in 2014), but it's certainly worth fixing.

We only warn when we were failing to allocate another field into the
tail padding of the newly aggregate class; this is the only ABI impact.

This also changes end_of_class to consider all data members, not just empty
data members; that used to be an additional flag, removed in r9-5710, but I
don't see any reason not to always include them.  This makes the result of
the function correspond to the ABI nvsize term and its nameless counterpart
that does include virtual bases.

When looking closely at other users of end_of_class, I realized that we were
assuming that the latter corresponded to the ABI dsize term, but it doesn't
if the class ends with an empty virtual base (in the rare case that the
empty base can't be assigned offset 0), and this matters for layout of
[[no_unique_address]].  So I added another mode that returns the desired
value for that case.  I'm not adding a warning for this ABI fix because it's
a C++20 feature.

	PR c++/103681

gcc/ChangeLog:

	* common.opt (fabi-version): Add v17.

gcc/cp/ChangeLog:

	* cp-tree.h (struct lang_type): Add non_pod_aggregate.
	(CLASSTYPE_NON_POD_AGGREGATE): New.
	* class.c (check_field_decls): Set it.
	(check_bases_and_members): Check it.
	(check_non_pod_aggregate): New.
	(enum eoc_mode): New.
	(end_of_class): Always include non-empty fields.
	Add eoc_nv_or_dsize mode.
	(include_empty_classes, layout_class_type): Adjust.

gcc/c-family/ChangeLog:

	* c-opts.c (c_common_post_options): Update defaults.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/macro0.C: Update value.
	* g++.dg/abi/no_unique_address6.C: New test.
	* g++.dg/abi/nsdmi-aggr1.C: New test.
	* g++.dg/abi/nsdmi-aggr1a.C: New test.