[Ada] Refine types of local variables in analysis of expression functions

[Pierre-Marie de Rodat]

Code cleanup related to handing of static expression functions in
GNATprove; behaviour is unaffected.

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

gcc/ada/

        * sem_ch6.adb (Analyze_Expression_Function): Change types local
        variables from Entity_Id to Node_Id.
diff --git a/gcc/ada/sem_ch6.adb b/gcc/ada/sem_ch6.adb
--- a/gcc/ada/sem_ch6.adb
+++ b/gcc/ada/sem_ch6.adb
@@ -640,9 +640,9 @@ package body Sem_Ch6 is
       if Present (Parameter_Specifications (New_Spec)) then
          declare
             Form_New_Def  : Entity_Id;
-            Form_New_Spec : Entity_Id;
+            Form_New_Spec : Node_Id;
             Form_Old_Def  : Entity_Id;
-            Form_Old_Spec : Entity_Id;
+            Form_Old_Spec : Node_Id;
 
          begin
             Form_New_Spec := First (Parameter_Specifications (New_Spec));