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-inline.hh

@@ -93,20 +93,12 @@ void EvalState::forceValue(Value & v, const PosIdx pos)
             expr->eval(*this, *env, v);
         } catch (...) {
             v.mkThunk(env, expr);
+            tryFixupBlackHolePos(v, pos);
             throw;
         }
     }
-    else if (v.isApp()) {
-        try {
-            callFunction(*v.app.left, *v.app.right, v, noPos);
-        } catch (InfiniteRecursionError & e) {
-            // only one black hole can *throw* in any given eval stack so we need not
-            // check whether the position is set already.
-            if (v.isBlackhole())
-                e.err.errPos = positions[pos];
-            throw;
-        }
-    }
+    else if (v.isApp())
+        callFunction(*v.app.left, *v.app.right, v, pos);
 }