This patch implements the declarations of private methods, the access
of private methods would be left to a future patch.
When a private methods declaration is encountered, we now:

- Create a brand symbol during class evaluation and store it in the
  context.
- Create the closures for the private methods
- Load the brand from the context and store it in the instance in the
  constructor.

Design: https://docs.google.com/document/d/1T-Ql6HOIH2U_8YjWkwK2rTfywwb7b3Qe8d3jkz72KwA/edit#

Bug: v8:8330
Change-Id: I2d695cbdc8a7367ddc7620d627b318f779d36150
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1568708
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61387}