Add optional deep chain with private access

This was also recently approved in TC39. I collected the results by taking the maximum of the optional chain and private instance fields results, then manually verifying the supported browsers. Only Chrome has it so far.

Re: compat-table#1612

data-esnext.js

@@ -929,6 +929,32 @@ exports.tests = [
         typescript3_8corejs3: false,
       }
     },
+    {
+      name: 'optional deep private instance class fields access',
+      exec: function () {/*
+        class C {
+          #x = 42;
+          x(o = {p: this}){
+            return o?.p.#x;
+          }
+        }
+        return new C().x() === 42 && new C().x(null) === void 0;
+      */},
+      res: {
+        ie11: false,
+        firefox2: false,
+        firefox74: firefox.privateClassFields,
+        chrome1: false,
+        chrome78: {val: 'flagged', note_id: "chrome-optional-chaining", note_html: "The feature have to be enabled via <code>--js-flags=\"--harmony-optional-chaining\"</code> flag"},
+        chrome80: true,
+        safari1: false,
+        safari13_1: false,
+        opera10_50: false,
+        graalvm20: false,
+        babel7corejs3: false,
+        typescript3_8corejs3: false,
+      }
+    },
     {
       name: 'computed instance class fields',
       exec: function () {/*