use singleton expr to generate black hole errors

this also reduces forceValue code size and removes the need for
hideInDiagnostics. coopting thunk forcing like this has the additional
benefit of clarifying how these errors can happen in the first place.

src/libexpr/eval.hh

@@ -77,14 +77,6 @@ struct PrimOp
      */
     std::optional<ExperimentalFeature> experimentalFeature;
 
-    /**
-     * Whether to hide this primop in diagnostics.
-     *
-     * Used to hide the fact that black holes are primop applications from
-     * stack traces.
-     */
-    bool hideInDiagnostics;
-
     /**
      * Validity check to be performed by functions that introduce primops,
      * such as RegisterPrimOp() and Value::mkPrimOp().
@@ -473,6 +465,8 @@ public:
      */
     inline void forceValue(Value & v, const PosIdx pos);
 
+    void tryFixupBlackHolePos(Value & v, PosIdx pos);
+
     /**
      * Force a value, then recursively force list elements and
      * attributes.