getOwnPropertyDescriptor return should allow undefined

getOwnPropertyDescriptor _should_ return undefined in certain situations. This is documented here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler/getOwnPropertyDescriptor

This change addresses #14923 and #10904. The original fix to 14923 was to the /lib/lib.es2015.proxy.d.ts file instead of /src/lib/lib.es2015.proxy.d.ts (see: https://github.com/Microsoft/TypeScript/pull/10550/commits). I think this meant that the change got over-written by a generation script (see note here: https://github.com/Microsoft/TypeScript/tree/master/lib)

Author: astegmaier

src/lib/es2015.proxy.d.ts

@@ -3,7 +3,7 @@ interface ProxyHandler<T> {
     setPrototypeOf? (target: T, v: any): boolean;
     isExtensible? (target: T): boolean;
     preventExtensions? (target: T): boolean;
-    getOwnPropertyDescriptor? (target: T, p: PropertyKey): PropertyDescriptor;
+    getOwnPropertyDescriptor? (target: T, p: PropertyKey): PropertyDescriptor | undefined;
     has? (target: T, p: PropertyKey): boolean;
     get? (target: T, p: PropertyKey, receiver: any): any;
     set? (target: T, p: PropertyKey, value: any, receiver: any): boolean;