[Ada] ICE handling discriminant-dependent index constraint for access component

The compiler would fail with an internal error in some cases involving
a discriminated record type that provides a discriminant-dependent index
constraint for the subtype of a component of an access-to-array type when a
dereference of that component of some object is mentioned in a pre- or
postcondition expression.

gcc/ada/

	* sem_ch4.adb
	(Analyze_Selected_Component): Define new Boolean-valued function,
	Constraint_Has_Unprefixed_Discriminant_Reference, which takes a
	subtype that is subject to a discriminant-dependent constraint and
	returns True if any of the constraint values are unprefixed
	discriminant names. Usually, the Etype of a selected component
	node is set to Etype of the component. However, in the case of an
	access-to-array component for which this predicate returns True,
	we instead use the base type of the Etype of the component.
	Normally such problematic discriminant references are addressed by
	calling Build_Actual_Subtype_Of_Component, but that doesn't work
	if Full_Analyze is False.

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