[Ada] Add -gnatX support for casing on discriminated values

gcc/ada/

	* exp_ch5.adb (Expand_General_Case_Statement): Add new function
	Else_Statements to handle the case of invalid data analogously
	to how it is handled when casing on a discrete value.
	* sem_case.adb (Has_Static_Discriminant_Constraint): A new
	Boolean-valued function.
	(Composite_Case_Ops.Scalar_Part_Count): Include discriminants
	when traversing components.
	(Composite_Case_Ops.Choice_Analysis.Traverse_Discrete_Parts):
	Include discriminants when traversing components; the component
	range for a constrained discriminant is a single value.
	(Composite_Case_Ops.Choice_Analysis.Parse_Choice): Eliminate
	Done variable and modify how Next_Part is computed so that it is
	always correct (as opposed to being incorrect when Done is
	True).  This includes changes in Update_Result (a local
	procedure).  Add new local procedure
	Update_Result_For_Box_Component and call it not just for box
	components but also for "missing" components (components
	associated with an inactive variant).
	(Check_Choices.Check_Composite_Case_Selector.Check_Component_Subtype):
	Instead of disallowing all discriminated component types, allow
	those that are unconstrained or statically constrained. Check
	discriminant subtypes along with other component subtypes.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Update
	documentation to reflect current implementation status.
	* gnat_rm.texi: Regenerate.

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