Since we are not using CodeAssemblerVariables anymore, we can properly
scope the class so that the only way to instance one is through the
typed version (i.e TypedCodeAssemblerVariable).

Bug: v8:6949
Change-Id: I3ee99bcbb9421aadd0944af57f6452e832719758
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2290858
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68797}