Fix wrong array type conversion with different storage orde

When two arrays of scalars have a different storage order in Ada, the
front-end makes sure that the conversion is performed component-wise
so that each component can be reversed.  So it's a little bit counter
productive that the ldist pass performs the opposite transformation
and synthesizes a memcpy/memmove in this case.

gcc/
	* tree-loop-distribution.cc (loop_distribution::classify_builtin_ldst):
	Bail out if source and destination do not have the same storage order.

gcc/testsuite/
	* gnat.dg/sso18.adb: New test.