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

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

gcc/ada/

	* exp_ch5.adb (Expand_General_Case_Statement.Pattern_Match): Add
	new function Indexed_Element to handle array element
	comparisons. Handle case choices that are array aggregates,
	string literals, or names denoting constants.
	* sem_case.adb (Composite_Case_Ops.Array_Case_Ops): New package
	providing utilities needed for casing on arrays.
	(Composite_Case_Ops.Choice_Analysis): If necessary, include
	array length as a "component" (like a discriminant) when
	traversing components. We do not (yet) partition choice analysis
	to deal with unequal length choices separately. Instead, we
	embed everything in the minimum-dimensionality Cartesian product
	space needed to handle all choices properly; this is determined
	by the length of the longest choice pattern.
	(Composite_Case_Ops.Choice_Analysis.Traverse_Discrete_Parts):
	Include length as a "component" in the traversal if necessary.
	(Composite_Case_Ops.Choice_Analysis.Parse_Choice.Traverse_Choice):
	Add support for case choices that are string literals or names
	denoting constants.
	(Composite_Case_Ops.Choice_Analysis): Include length as a
	"component" in the analysis if necessary.
	(Check_Choices.Check_Case_Pattern_Choices.Ops.Value_Sets.Value_Index_Count):
	Improve error message when capacity exceeded.
	* doc/gnat_rm/implementation_defined_pragmas.rst: Update
	documentation to reflect current implementation status.
	* gnat_rm.texi: Regenerate.