Fix misoptimization of CONSTRUCTOR with reverse SSO

Fix misoptimization of CONSTRUCTOR with reverse SSO

fold_ctor_reference already punts on a CONSTRUCTOR whose type has reverse
storage order, but it can be invoked in a couple of places on a CONSTRUCTOR
with native storage order that has been wrapped in a VIEW_CONVERT_EXPR to a
type with reverse storage order; this would require a post adjustment that
does not currently exist, thus yield wrong code for this admittedly quite
pathological (but supported) case.

gcc/
	* gimple-fold.cc (fold_const_aggregate_ref_1) <COMPONENT_REF>:
	Bail out immediately if the reference has reverse storage order.
	* tree-ssa-sccvn.cc (fully_constant_vn_reference_p): Likewise.
gcc/testsuite/
	* gnat.dg/sso20.adb: New test.