[Ada] Accept aspect Yield on subprogram bodies acting as specs

[Pierre-Marie de Rodat via Gcc-patches]

A small fix for the aspect Yield defined in AI12-0279 for Ada 2022, to
accept aspect given for a subprogram body which acts as its own spec.

For example:

   procedure Switch with Yield => True is
   begin
      ...
   end Switch;

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

gcc/ada/

        * sem_ch13.adb (Analyze_Aspect_Yield): Look at the entity kind,
        not at the declaration kind.
diff --git a/gcc/ada/sem_ch13.adb b/gcc/ada/sem_ch13.adb
--- a/gcc/ada/sem_ch13.adb
+++ b/gcc/ada/sem_ch13.adb
@@ -2724,13 +2724,11 @@ package body Sem_Ch13 is
                Expr_Value : Boolean := False;
 
             begin
-               --  Check valid declarations for 'Yield
+               --  Check valid entity for 'Yield
 
-               if Nkind (N) in N_Abstract_Subprogram_Declaration
-                             | N_Entry_Declaration
-                             | N_Generic_Subprogram_Declaration
-                             | N_Subprogram_Declaration
-                             | N_Formal_Subprogram_Declaration
+               if (Is_Subprogram (E)
+                     or else Is_Generic_Subprogram (E)
+                     or else Is_Entry (E))
                  and then not Within_Protected_Type (E)
                then
                   null;