This patch implements the access of private methods:

- When building property loads, check whether it requires
  a brand check. If so, build the brand check and load the
  property (the method) from the context instead.
- Throw type errors when there is an attempted write to private
  methods.

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

Bug: v8:8330
Change-Id: Ic917d2a0030196c1940b0c0ba65a340af736c769
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1610383
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62292}