SC_Obligations: assert that scope entities are set only once per CU

Instrumenters are not supposed to set scope entities once, and then
override them later for the same CU: ensure this with an assertion.

Author: pmderodat

tools/gnatcov/sc_obligations.adb

@@ -4362,9 +4362,15 @@ package body SC_Obligations is
    ------------------------
 
    procedure Set_Scope_Entities
-     (CU : CU_Id; Scope_Entities : Scope_Entities_Trees.Tree) is
+     (CU : CU_Id; Scope_Entities : Scope_Entities_Trees.Tree)
+   is
+      SE : Scope_Entities_Trees.Tree renames
+        CU_Vector.Reference (CU).Scope_Entities;
    begin
-      CU_Vector.Reference (CU).Scope_Entities := Scope_Entities;
+      --  Scopes are supposed to be set only once per compilation unit
+
+      pragma Assert (SE.Is_Empty);
+      SE := Scope_Entities;
    end Set_Scope_Entities;
 
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