middle-end: Reject flexible array members in __builtin_clear_padding [PR97943]

middle-end: Reject flexible array members in __builtin_clear_padding [PR97943]

As mentioned in the PR, we currently ICE on flexible array members in
structs and unions during __builtin_clear_padding processing.

Jason said in the PR he'd prefer an error in these cases over forcefully
handling it as [0] arrays (everything is padding then) or consider the
arrays to have as many whole elements as would fit into the tail padding.

So, this patch implements that.

2020-11-25  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/97943
	* gimple-fold.c (clear_padding_union, clear_padding_type): Error on and
	ignore flexible array member fields.  Ignore fields with
	error_mark_node type.

	* c-c++-common/builtin-clear-padding-2.c: New test.
	* c-c++-common/builtin-clear-padding-3.c: New test.
	* g++.dg/ext/builtin-clear-padding-1.C: New test.
	* gcc.dg/builtin-clear-padding-2.c: New test.