[parser] add source posisionts to private field synthetic variables

Bug: v8:8354
Change-Id: I20bb169695d7ecca739f14f9a3ddfafed0dcb964
Reviewed-on: https://chromium-review.googlesource.com/c/1393284Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#58652}

src/parsing/parser.cc

@@ -3038,6 +3038,7 @@ void Parser::DeclareClassField(ClassLiteralProperty* property,
     class_info->properties->Add(property, zone());
   } else if (is_private) {
     Variable* private_name_var = CreateSyntheticContextVariable(property_name);
+    private_name_var->set_initializer_position(property->value()->position());
     property->set_private_name_var(private_name_var);
     class_info->properties->Add(property, zone());
   }

test/message/fail/class-fields-private-source-positions.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --harmony-private-fields
+
+var o = {};
+class C {
+    #a = 0;
+
+    [o.#a](){}
+}

test/message/fail/class-fields-private-source-positions.out

+*%(basename)s:11: TypeError: Read of private field #a from an object which did not contain the field
+    [o.#a](){}
+       ^
+TypeError: Read of private field #a from an object which did not contain the field
+    at *%(basename)s:11:8