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

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

[Pierre-Marie de Rodat]

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Improve existing support for the Ada extension feature of casing on
composite values to handle casing on array values; in particular, casing
on an array value whose subtype is unconstrained (or dynamically
constrained) so that choices in the case statement may have differing
lengths. This commit does not include support for pattern-bound
identifiers in such cases.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.

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