Fix couple of endianness issues in fold_ctor_reference

fold_ctor_reference attempts to use a recursive local processing in order
to call native_encode_expr on the leaf nodes of the constructor, before
falling back to calling native_encode_initializer if this fails.

There are a couple of issues related to endianness present in it:
  1) it does not specifically handle integral bit-fields; now these are left
justified on big-endian platforms so cannot be treated like ordinary fields.
  2) it does not check that the constructor uses the native storage order.

gcc/
	* gimple-fold.cc (fold_array_ctor_reference): Fix head comment.
	(fold_nonarray_ctor_reference): Likewise.  Specifically deal
	with integral bit-fields.
	(fold_ctor_reference): Make sure that the constructor uses the
	native storage order.

gcc/testsuite/
	* gcc.c-torture/execute/20230630-1.c: New test.
	* gcc.c-torture/execute/20230630-2.c: Likewise.
	* gcc.c-torture/execute/20230630-3.c: Likewise
	* gcc.c-torture/execute/20230630-4.c: Likewise