c++: [[no_unique_address]] fixes. [PR96105]

c++: [[no_unique_address]] fixes. [PR96105]

We were wrongly checking is_empty_class on the result of strip_array_types
rather than the actual field type.  We weren't considering the alignment of
the data member.  We needed to handle unions the same way as
layout_nonempty_base_or_field.

gcc/cp/ChangeLog:

	PR c++/96105
	PR c++/96052
	PR c++/95976
	* class.c (check_field_decls): An array of empty classes is not an
	empty data member.
	(layout_empty_base_or_field): Handle explicit alignment.
	Fix union handling.

gcc/testsuite/ChangeLog:

	PR c++/96105
	PR c++/96052
	PR c++/95976
	* g++.dg/cpp2a/no_unique_address4.C: New test.
	* g++.dg/cpp2a/no_unique_address5.C: New test.
	* g++.dg/cpp2a/no_unique_address6.C: New test.