Adapt to libadalang API change

In part of the work to support the GNAT experimental feature allowing
to have fixed lower bounds for array types and subtypes in libadalang,
`UnconstrainedArrayIndex.F_Subtype_Indication` (an Expr node) has been
renamed to `UnconstrainedArrayIndex.F_Subtype_Name` (a Name node) in
order to fix a bug in the Ada grammar description of libadalang.

src/tgen/tgen-types-translation.adb

@@ -1444,7 +1444,8 @@ package body TGen.Types.Translation is
             declare
                Index_Type : constant Translation_Result :=
                  Translate
-                   (Index.F_Subtype_Indication.As_Type_Expr, Verbose_Diag);
+                   (Index.F_Subtype_Name.As_Name.P_Name_Designated_Type,
+                    Verbose_Diag);
             begin
                if Index_Type.Success then
                   Res_Typ.Index_Types (Current_Index_Type) := Index_Type.Res;