vm: Function declarations should use [[Define]] instead of [[Set]] on the global object

- **Version**: v13.8.0 - **Platform**: All; tested on **Travis CI** (**Ubuntu 16.04.6 LTS**) - **Subsystem**: vm ## What steps will reproduce the bug? ```js const vm = require("vm"); let impl = { status: "" }; let ctx = vm.createContext(); Object.defineProperty(ctx, "status", { configurable: true, enumerable: true, get() { return impl.status; }, set(v) { impl.status = `${v}`; }, }); // https://github.com/web-platform-tests/wpt/blob/master/xhr/status-async.htm vm.runInContext(` function statusRequest(...args) { console.log(args.map(JSON.stringify).join("\t")); } function status(code, text, content, type) { statusRequest("GET", code, text, content, type); statusRequest("HEAD", code, text, content, type); statusRequest("CHICKEN", code, text, content, type); } status(204, "UNICORNSWIN", "", "") status(401, "OH HELLO", "Not today.", "") status(402, "FIVE BUCKS", "<x>402<\/x>", "text/xml") status(402, "FREE", "Nice!", "text/doesnotmatter") status(402, "402 TEH AWESOME", "", "") status(502, "YO", "", "") status(502, "lowercase", "SWEET POTATO", "text/plain") status(503, "HOUSTON WE HAVE A", "503", "text/plain") status(699, "WAY OUTTA RANGE", "699", "text/plain") `, ctx); ``` ## How often does it reproduce? Is there a required condition? Always. ## What is the expected behavior? The functions get defined using `[[DefineOwnProperty]]` as specified in [ECMA‑262 § 8.1.1.4.18 `CreateGlobalFunctionBinding`](https://tc39.es/ecma262/#sec-createglobalfunctionbinding). <details> <summary><strong>Expected log output</strong></summary> <pre><table><tr><td>"GET"</td> <td>204</td> <td>"UNICORNSWIN"</td> <td>""</td> <td>""</td></tr> <tr><td>"HEAD"</td> <td>204</td> <td>"UNICORNSWIN"</td> <td>""</td> <td>""</td></tr> <tr><td>"CHICKEN"</td> <td>204</td> <td>"UNICORNSWIN"</td> <td>""</td> <td>""</td></tr> <tr><td>"GET"</td> <td>401</td> <td>"OH HELLO"</td> <td>"Not today."</td> <td>""</td></tr> <tr><td>"HEAD"</td> <td>401</td> <td>"OH HELLO"</td> <td>"Not today."</td> <td>""</…