fix(es/helpers): Fix loose mode (#6691)

**Related issue:**

 - Closes https://github.com/swc-project/swc/issues/6581.

crates/swc/tests/exec/issues-6xxx/6581/1/.swcrc

@@ -0,0 +1,19 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "typescript",
+            "tsx": true
+        },
+        "target": "es2019",
+        "loose": true,
+        "minify": {
+            "compress": false,
+            "mangle": false
+        }
+    },
+    "module": {
+        "type": "es6"
+    },
+    "minify": false,
+    "isModule": true
+}

crates/swc/tests/exec/issues-6xxx/6581/1/exec.ts

@@ -0,0 +1,9 @@
+class Hoge {
+    #hoge: number
+    constructor() {
+        this.#hoge = 42
+    }
+}
+
+new Hoge()
+new Hoge()

crates/swc/tests/exec/issues-6xxx/6581/2/.swcrc

@@ -0,0 +1,20 @@
+{
+    "jsc": {
+        "parser": {
+            "syntax": "typescript",
+            "tsx": true
+        },
+        "target": "es2019",
+        "loose": true,
+        "minify": {
+            "compress": false,
+            "mangle": false
+        },
+        "externalHelpers": true
+    },
+    "module": {
+        "type": "es6"
+    },
+    "minify": false,
+    "isModule": true
+}

crates/swc/tests/exec/issues-6xxx/6581/2/exec.ts

@@ -0,0 +1,9 @@
+class Hoge {
+    #hoge: number
+    constructor() {
+        this.#hoge = 42
+    }
+}
+
+new Hoge()
+new Hoge()

packages/swc-helpers/src/_class_private_field_loose_base.mjs

@@ -3,5 +3,5 @@ export default function _classPrivateFieldBase(receiver, privateKey) {
     throw new TypeError("attempted to use private field on non-instance");
   }
 
-  return receiver;
+  return receiver[privateKey];
 }