gimple-fold: Fix another __builtin_clear_padding ICE

gimple-fold: Fix another __builtin_clear_padding ICE

When playing with __builtin_bit_cast, I have noticed __builtin_clear_padding
ICE on the G class below.  The artificial field with D type has offset 0
and size 8 bytes, but the following artificial field with E type has offset
0 and size 0, so it triggers the asserts that we don't move current position
backwards.  Fixed by ignoring is_empty_type (TREE_TYPE (field)) fields, all
of their bits are padding which is what is added when skipping over to next
field anyway.

2020-11-27  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/88101
	* gimple-fold.c (clear_padding_type): Ignore fields with is_empty_type
	types.

	* g++.dg/torture/builtin-clear-padding-3.C: New test.