c++: Treat unnamed bitfields as padding for __has_unique_object_representations ...

c++: Treat unnamed bitfields as padding for __has_unique_object_representations [PR109096]

As reported in the PR, for __has_unique_object_representations we
were treating unnamed bitfields as named ones, which is wrong, they
are actually padding.

THe following patch fixes that.

2023-03-14  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109096
	* tree.cc (record_has_unique_obj_representations): Ignore unnamed
	bitfields.

	* g++.dg/cpp1z/has-unique-obj-representations3.C: New test.

(cherry picked from commit c35cf160a0ed81570cff6600dba465cf95fa80fa)