[Ada] Create internal type for null array aggregate as an itype

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

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Internal type created for the null array aggregate of Ada 2022 was
created as a temporary entity and then flagged as internal, but it is
better to create this type directly as an itype.

In particular, when the null array aggregate appears in a spec
expression, its type declaration will not be attached to the AST.
An itype will have Associated_Node_For_Itype, so that the context of
the type can be recovered, which is what GNATprove does.

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

gcc/ada/

	* sem_aggr.adb (Resolve_Null_Array_Aggregate): Create internal
	type for the aggregate as an itype.

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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
@@ -4088,7 +4088,10 @@ package body Sem_Aggr is
       Index  : Node_Id;
       Lo, Hi : Node_Id;
       Constr : constant List_Id := New_List;
-      Subt   : constant Entity_Id := Make_Temporary (Loc, 'S');
+      Subt   : constant Entity_Id :=
+        Create_Itype (Ekind       => E_Array_Subtype,
+                      Related_Nod => N,
+                      Suffix      => 'S');
 
    begin
       --  Create a constrained subtype with null dimensions
@@ -4134,7 +4137,6 @@ package body Sem_Aggr is
                       Make_Index_Or_Discriminant_Constraint (Loc, Constr)));
 
       Insert_Action (N, Decl);
-      Set_Is_Internal (Subt);
       Analyze (Decl);
       Set_Etype (N, Subt);
       Set_Compile_Time_Known_Aggregate (N);