We have hundreds of classes that derive from {BitField} without adding
any functionality. This CL switches all such occurrences to 'using'
declarations instead.

Before:
  class MyBitField : public BitField<int, 6, 4, MyEnum> {};
After:
  using MyBitField = BitField<int, 6, 4, MyEnum>;

This might reduce compilation time by reducing the number of existing
classes.

The old pattern is forbidden now by making {BitField} final.

R=yangguo@chromium.org

Bug: v8:9396, v8:7629
Change-Id: I8a8364707e8eae0bb522af2459c160e3293eecbb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1722565Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62956}