LibJS: Include the WeakMap key (not value) in non-weakable key errors

It is the key that cannot be held weakly, and thus the key that should
be include in the error message.

Libraries/LibJS/Runtime/WeakMapPrototype.cpp

@@ -115,7 +115,7 @@ JS_DEFINE_NATIVE_FUNCTION(WeakMapPrototype::set)
 
     // 3. If CanBeHeldWeakly(key) is false, throw a TypeError exception.
     if (!can_be_held_weakly(key))
-        return vm.throw_completion<TypeError>(ErrorType::CannotBeHeldWeakly, value.to_string_without_side_effects());
+        return vm.throw_completion<TypeError>(ErrorType::CannotBeHeldWeakly, key);
 
     // 4. For each Record { [[Key]], [[Value]] } p of M.[[WeakMapData]], do
     //    a. If p.[[Key]] is not empty and SameValue(p.[[Key]], key) is true, then

Libraries/LibJS/Tests/builtins/WeakMap/WeakMap.prototype.set.js

@@ -14,10 +14,12 @@ test("basic functionality", () => {
 
 test("invalid values", () => {
     const weakMap = new WeakMap();
-    [-100, Infinity, NaN, "hello", 152n].forEach(value => {
+    [-100, Infinity, NaN, "hello", 152n].forEach(key => {
+        const suffix = typeof key === "bigint" ? "n" : "";
+
         expect(() => {
-            weakMap.set(value, value);
-        }).toThrowWithMessage(TypeError, "cannot be held weakly");
+            weakMap.set(key, 1);
+        }).toThrowWithMessage(TypeError, `${key}${suffix} cannot be held weakly`);
     });
 });