[Ada] Fix insertion of a runtime check for null array aggregate

["Marc Poulhiès" <poulhies@adacore.com>]

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A null array aggregate of Ada 2022 requires a conditional runtime check
that was inserted as an if-statement. While gigi can handle statements
inserted into a list of declarations, in GNATprove such a statement will
cause a crash. It is better to insert a conditional raise node, which is
properly handled by both gigi and GNATprove.

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

gcc/ada/

	* sem_aggr.adb (Resolve_Null_Array_Aggregate): Insert check as a
	Raise_Constraint_Error node and not an If_Statement.

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diff --git a/gcc/ada/sem_aggr.adb b/gcc/ada/sem_aggr.adb
--- a/gcc/ada/sem_aggr.adb
+++ b/gcc/ada/sem_aggr.adb
@@ -4113,12 +4113,10 @@ package body Sem_Aggr is
          --  including modular).
 
          Check :=
-           Make_If_Statement (Loc,
+           Make_Raise_Constraint_Error (Loc,
              Condition =>
                Make_Op_Le (Loc, New_Copy_Tree (Lo), New_Copy_Tree (Hi)),
-             Then_Statements =>
-               New_List (Make_Raise_Constraint_Error
-                           (Loc, Reason => CE_Range_Check_Failed)));
+             Reason => CE_Range_Check_Failed);
 
          Insert_Action (N, Check);