This patch fixes a bug in Is_Reachable, which causes a crash when checks
are on.

2019-08-12  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/a-cbmutr.adb (Is_Reachable): Declare Idx to be of the
	base subtype.  Clearly it makes no sense to loop "while Idx >=
	0", if Idx is of a nonnegative subtype.

From-SVN: r274301