C: Flex array in the middle via type alias is not reported [PR120353]

C: Flex array in the middle via type alias is not reported [PR120353]

The root cause of the bug is: the TYPE_INCLUDES_FLEXARRAY marking of the
structure type is not copied to its aliased type.
The fix is to copy this marking to all the variant types of the current
structure type.

	PR c/120353

gcc/c/ChangeLog:

	* c-decl.cc (finish_struct): Copy TYPE_INCLUDES_FLEXARRAY marking
	to all the variant types of the current structure type.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr120353.c: New test.