Passing TYPE_SIZE_UNIT of the element as the 6th argument to .ACCESS_WITH_SIZE (PR121000)

Passing TYPE_SIZE_UNIT of the element as the 6th argument to .ACCESS_WITH_SIZE (PR121000)

The size of the element of the FAM _cannot_ reliably depends on the original
TYPE of the FAM that we passed as the 6th parameter to the .ACCESS_WITH_SIZE:

     TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (gimple_call_arg (call, 5))))

when the element of the FAM has a variable length type. Since the variable
 that represents TYPE_SIZE_UNIT has no explicit usage in the original IL,
compiler transformations (such as DSE) that are applied before object_size
phase might eliminate the whole definition to the variable that represents
the TYPE_SIZE_UNIT of the element of the FAM.

In order to resolve this issue, instead of passing the original TYPE of the
FAM as the 6th argument to .ACCESS_WITH_SIZE, we should explicitly pass the
original TYPE_SIZE_UNIT of the element TYPE of the FAM as the 6th argument
to the call to  .ACCESS_WITH_SIZE.

	PR middle-end/121000

gcc/c/ChangeLog:

	* c-typeck.cc (build_access_with_size_for_counted_by): Update comments.
	Pass TYPE_SIZE_UNIT of the element as the 6th argument.

gcc/ChangeLog:

	* internal-fn.cc (expand_ACCESS_WITH_SIZE): Update comments.
	* internal-fn.def (ACCESS_WITH_SIZE): Update comments.
	* tree-object-size.cc (access_with_size_object_size): Update comments.
	Get the element_size from the 6th argument directly.

gcc/testsuite/ChangeLog:

	* gcc.dg/flex-array-counted-by-pr121000.c: New test.