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

Bug: v8:7703, v8:9183
Change-Id: I388b008aad0dbeb2d33fc5fb80c5f29b55ef993e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1588419
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61441}