This patch adds a new assign type `PRIVATE_METHOD`. We now use this
for private method references in the form `obj.#key` when `#key`
resolves to a private method.

To obtain the type of the key variables after scope analysis, this
patch add a bit to Variable to recognize private method variables
whose load requires a brand check.

Also renamed `PropertyExpressionWithPrivateFieldKey` in ExpressionType
to `PrivateReference` and added `PRIVATE_CALL` to `CallType` - we'll
use the new types later when we implement private methods, which
require special brand checking semantics to load methods directly
from the context instead of from the object in order to save memory.

Bug: v8:8330
Change-Id: Idc1dcd4d514c1b3f8a31c99e49e34249449f0677
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1642772
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62255}