This is a prototype implementation of the feature for defining array
types and subtypes with fixed lower bounds for index ranges as proposed
as part of the "Inox" language design effort. This feature is currently
supported under control of the -gnatX (language extensions) switch, and
allows unconstrained array types as well as unconstrained subtypes to be
declared with indexes that use the special syntax "expression .. <>", to
indicate that the lower bound of the array type or subtype is fixed at a
value given by an expression. In contexts such as actual parameters,
object declarations, return statements, and type conversions, if the
context is such an unconstrained array subtype with a fixed lower bound,
bounds sliding is performed. Also, in certain contexts, such as index
constraints, consistency of the range imposed by the constraint with the
fixed lower bound of the array type being constrained is enforced.

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

gcc/ada/

	* checks.adb (Discrete_Range_Cond): For an index subtype that
	has a fixed lower bound, require that the range's lower bound
	match that of the subtype.
	(Selected_Range_Checks): Warn about the case where a static
	lower bound does not equal an index subtype's fixed lower bound.
	* einfo.ads (Is_Fixed_Lower_Bound_Array_Subtype,
	Is_Fixed_Lower_Bound_Index_Subtype): Document new entity flag.
	* exp_ch4.adb (Expand_N_Type_Conversion): If the operand is of
	an unconstrained array subtype with fixed lower bound, then
	Expand_Sliding_Conversion is applied to the operand.
	* exp_ch6.adb (Expand_Simple_Function_Return): If the result
	subtype is an unconstrained array subtype with fixed lower
	bound, then Expand_Sliding_Conversion is applied to the return
	object.
	* exp_util.ads (Expand_Sliding_Conversion): New procedure for
	applying a sliding subtype conversion to an array object of a
	fixed-lower-bound subtype when needed.
	* exp_util.adb: Add with_clause for Freeze.
	(Expand_Sliding_Conversion): New procedure for applying a
	sliding subtype conversion to an array object of a
	fixed-lower-bound subtype when needed.  It traverses the indexes
	of the unconstrained array type/subtype to create a target
	constrained subtype and rewrites the array object to be a
	conversion to that subtype, when there's at least one index
	whose lower bound does not statically match the fixed-lower
	bound of the target subtype.
	* gen_il-fields.ads (type Opt_Field_Enum): Add literals
	Is_Fixed_Lower_Bound_Array_Subtype and
	Is_Fixed_Lower_Bound_Index_Subtype for new flags on type
	entities.
	* gen_il-gen-gen_entities.adb: Add calls to
	Create_Semantic_Field for the new fixed-lower-bound flags on
	type entities.
	* par-ch3.adb (P_Array_Type_Definition): Add handling for
	parsing of fixed-lower-bound index ranges in unconstrained array
	types. Report an error if such an index is encountered and GNAT
	language extensions are not enabled.
	(P_Index_Subtype_Def_With_Fixed_Lower_Bound): Support procedure
	for parsing unconstrained index ranges.
	(P_Index_Or_Discriminant_Constraint): Add handling for parsing
	of index constraints that specify ranges with fixed lower
	bounds. Report an error if such an index is encountered and GNAT
	language extensions are not enabled.
	* sem_ch3.adb (Analyze_Object_Declaration): If the object's
	nominal subtype is an array subtype with fixed lower bound, then
	Expand_Sliding_Conversion is applied to the object.
	(Array_Type_Declaration): Mark the array type and the subtypes
	of any indexes that specify a fixed lower bound as being
	fixed-lower-bound subtypes, and set the High_bound of the range
	of such an index to the upper bound of the named subtype.
	(Constrain_Array): For an array subtype with one or more index
	ranges specifying a fixed lower bound, set Is_Constrained to
	False and set the array subtype's
	Is_Fixed_Lower_Bound_Array_Subtype flag to True.
	(Constrain_Index): Mark the subtypes of an index that specifies
	a fixed lower bound as being a fixed-lower-bound index subtype,
	and set the High_bound of the range of such an index to the
	upper bound of the base type of the array type's corresponding
	index.
	* sem_res.adb (Resolve_Actuals): If a formal is of an
	unconstrained array subtype with fixed lower bound, then
	Expand_Sliding_Conversion is applied to the actual.
	* sem_util.adb (Build_Actual_Subtype): If the actual subtype
	corresponds to an unconstrained array subtype having any indexes
	with fixed lower bounds, then set the lower bounds of any such
	indexes of the actual subtype to the appropriate fixed lower
	bound of the formal subtype (rather than taking it from the
	formal itself).
	* sprint.adb (Sprint_Node_Actual, case N_Range): If a range's
	Etype has a fixed lower bound, then print "<>" rather than the
	High_Bound of the range.