This patch adds:

- VariableMode::kPrivateMethod
- VariableMode::kPrivateSetterOnly
- VariableMode::kPrivateGetterOnly
- VariableMode::kPrivateGetterAndSetter

And replace the previous RequiresBrandCheckFlag by inferring
whether the brand check is required from these VariableModes.
It is then possible to check duplicate non-complementary
accessors in the parsers and throw early errors, and allow
complementary accessors to be associated with the same
private name variable.

This patch also adds the following AssignType:

- PRIVATE_METHOD
- PRIVATE_GETTER_ONLY
- PRIVATE_SETTER_ONLY
- PRIVATE_GETTER_AND_SETTER

corresponding to the new VariableModes so that it's possible
to generate specialized code for different type of
private accessor declarations.

Design doc: https://docs.google.com/document/d/10W4begYfs7lmldSqBoQBBt_BKamgT8igqxF9u50RGrI/edit

Bug: v8:8330
Change-Id: I0fb61b1be248630d1eadd74fb16d7d64a421f4c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1695204
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62988}