C: Flex array in union followed by a structure field is not reported [PR120354]

C: Flex array in union followed by a structure field is not reported [PR120354]

There is only one last_field for a structure type, but there might
be multiple last_fields for a union type, therefore we should ORed
the result of TYPE_INCLUDES_FLEXARRAY for multiple last_fields of
a union type.

	PR c/120354

gcc/c/ChangeLog:

	* c-decl.cc (finish_struct): Or the results for TYPE_INCLUDES_FLEXARRAY.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr120354.c: New test.

(cherry picked from commit 70418e6c0120cfce33ab69628602dfdadbed683a)