[Ada] Fix error message for import aspect

[Ada] Fix error message for import aspect

[Pierre-Marie de Rodat]

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Confusing error messages are emitted about arguments when erroneously
using the import/convention aspect.  This is caused by GNAT turning
aspects into pragmas and then emitting error messages about pragmas.
This is fixed by checking if the pragma comes from an aspect
specification and emitting a different error message if it does.

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

gcc/ada/

	* sem_prag.adb (Process_Convention,
	Process_Import_Or_Interface): Fix error message.

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diff --git a/gcc/ada/sem_prag.adb b/gcc/ada/sem_prag.adb
--- a/gcc/ada/sem_prag.adb
+++ b/gcc/ada/sem_prag.adb
@@ -8270,8 +8270,13 @@ package body Sem_Prag is
             --  Accept Intrinsic Export on types if Relaxed_RM_Semantics
 
             if not (Is_Type (E) and then Relaxed_RM_Semantics) then
-               Error_Pragma_Arg
-                 ("second argument of pragma% must be a subprogram", Arg2);
+               if From_Aspect_Specification (N) then
+                  Error_Pragma_Arg
+                     ("entity for aspect% must be a subprogram", Arg2);
+               else
+                  Error_Pragma_Arg
+                     ("second argument of pragma% must be a subprogram", Arg2);
+               end if;
             end if;
 
          --  Special checks for C_Variadic_n
@@ -9543,10 +9548,17 @@ package body Sem_Prag is
             Process_Import_Predefined_Type;
 
          else
-            Error_Pragma_Arg
-              ("second argument of pragma% must be object, subprogram "
-               & "or incomplete type",
-               Arg2);
+            if From_Aspect_Specification (N) then
+               Error_Pragma_Arg
+                  ("entity for aspect% must be object, subprogram "
+                     & "or incomplete type",
+                   Arg2);
+            else
+               Error_Pragma_Arg
+                  ("second argument of pragma% must be object, subprogram "
+                     & "or incomplete type",
+                   Arg2);
+            end if;
          end if;
 
          --  If this pragma applies to a compilation unit, then the unit, which