[Ada] Stronger assertion about flag for checking static expressions

[Pierre-Marie de Rodat]

Ensure that Checking_For_Potentially_Static_Expression flag is
manipulated in a stack-like manner (with stack depth 1 at the most). The
previous assertion didn't prevent us from setting the flag to True or to
False twice in a row.

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

gcc/ada/

        * sem_eval.adb (Set_Checking_Potentially_Static_Expression):
        Stronger assertion.
diff --git a/gcc/ada/sem_eval.adb b/gcc/ada/sem_eval.adb
--- a/gcc/ada/sem_eval.adb
+++ b/gcc/ada/sem_eval.adb
@@ -6481,11 +6481,10 @@ package body Sem_Eval is
 
    procedure Set_Checking_Potentially_Static_Expression (Value : Boolean) is
    begin
-      --  Verify that we're not currently checking for a potentially static
-      --  expression unless we're disabling such checking.
+      --  Verify that we only start/stop checking for a potentially static
+      --  expression and do not start or stop it twice in a row.
 
-      pragma Assert
-        (not Checking_For_Potentially_Static_Expression or else not Value);
+      pragma Assert (Checking_For_Potentially_Static_Expression /= Value);
 
       Checking_For_Potentially_Static_Expression := Value;
    end Set_Checking_Potentially_Static_Expression;