[FIR] Add toString() methods to some ConeSubstitutor implementation

This is needed only for pleasant debugging

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/calls/TypeVariableTypeRemovingSubstitutor.kt

@@ -30,4 +30,8 @@ private class TypeVariableTypeRemovingSubstitutor(typeContext: ConeTypeContext)
         }
         return ConeErrorType(ConeUnknownLambdaParameterTypeDiagnostic())
     }
+
+    override fun toString(): String {
+        return "{<Type variable> -> <Error type>}"
+    }
 }