Rename variables and flag names so that the classes can be reused
by private methods implementation.

In particular:

Rename "fields" to "members" in the initializer so that we can
initialize both fields and private methods/accessors there,
for example:

instance_fields_initializer -> instance_members_initializer
InitializeClassFieldsStatement -> InitializeClassMembersStatement

Rename "private field" to "private name" for the private symbols
used to implement private fields so that we can use them to
store private methods/accessors later as well, for example:

private_field_name_var -> private_name_var
NewPrivateFieldSymbol -> NewPrivateNameSymbol

The follow-on is in
https://chromium-review.googlesource.com/c/v8/v8/+/1301018

The design doc is in
https://docs.google.com/document/d/1T-Ql6HOIH2U_8YjWkwK2rTfywwb7b3Qe8d3jkz72KwA/edit?usp=sharing

Bug: v8:8330
Change-Id: I1cdca8def711da879b6e4d67c5ff0a5a4a36abbe
Reviewed-on: https://chromium-review.googlesource.com/c/1312597Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#57289}