[Ada] Accept calls to abstract subprograms in class-wide pre/post-conditions

[Ada] Accept calls to abstract subprograms in class-wide pre/post-conditions

Fix a regression in the support for Ada 2022's treatment of calls to
abstract subprograms in pre/post-conditions (thanks to Javier Miranda
for producing this patch).

gcc/ada/

	* sem_disp.adb (Check_Dispatching_Context): When checking to see
	whether an expression occurs in a class-wide pre/post-condition,
	also check for the possibility that it occurs in a class-wide
	preconditions subprogram that was introduced as part of
	expansion. Without this fix, some legal calls occuring in
	class-wide preconditions may be incorrectly flagged as violating
	the "a call to an abstract subprogram must be dispatching" rule.