Instead of exposing a {kNext} constant to be used to construct the next
bitfield, expose a templatized {Next} type alias. This ensures that the
storage type is the same for all bitfields created this way. It's also
shorter.

Apart from the expected changes in the code base, the AST node classes
are changed to expose a {NextBitField} templated type alias instead of
a {kNextBitFieldIndex} constant. They thus follow the same pattern as
{BitField} itself.

R=jkummerow@chromium.org, mstarzinger@chromium.org, verwaest@chromium.org

Bug: v8:9396
Change-Id: I70a1b0bd71cde694ec53444de0ca55e4cf0a3836
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1728615Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63068}