Add front end object references to back end objects, which requires
a significant amount of refactoring, because the back end objects
have to be constructed during the construction of the front end
objects, so that the references can be passed to the back end objects,
which then can be passed to the front end member initialization.
That would have been easier with inheritance than with PImpl.

Bug: angleproject:5904
Change-Id: Ib58e6a698e76987bdd63cd8088f923424d6c622b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2897249
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>