c++: Skip unnamed bit-fields more

c++: Skip unnamed bit-fields more

As Jason noticed in
<https://gcc.gnu.org/pipermail/gcc-patches/2021-November/583592.html>,
we shouldn't require an initializer for an unnamed bit-field, because,
as [class.bit] says, they cannot be initialized.

gcc/cp/ChangeLog:

	* class.c (default_init_uninitialized_part): Use
	next_initializable_field.
	* method.c (walk_field_subobs): Skip unnamed bit-fields.

gcc/testsuite/ChangeLog:

	* g++.dg/init/bitfield6.C: New test.