ubsan: Avoid -Wpadded warnings [PR94641]

-Wpadded warnings aren't really useful for the artificial types that GCC
lays out for ubsan.

2020-04-21  Jakub Jelinek  <jakub@redhat.com>

	PR c/94641
	* stor-layout.c (place_field, finalize_record_size): Don't emit
	-Wpadded warning on TYPE_ARTIFICIAL rli->t.
	* ubsan.c (ubsan_get_type_descriptor_type,
	ubsan_get_source_location_type, ubsan_create_data): Set
	TYPE_ARTIFICIAL.
	* asan.c (asan_global_struct): Likewise.

	* c-c++-common/ubsan/pr94641.c: New test.