This is a first step towards unification of Object and MaybeObject
definitions.

Having an TaggedImpl template will simplify adding compressed variants
of Object and MaybeObject which is required for avoiding unnecessary
value decompression in tight value copying loops and write barrier
implementations.

Bug: v8:7703, v8:9183
Change-Id: I4c1931c22359533d50cf4a2c7f1339dd55c0c707
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1588460Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61385}