Replace the Advance/Done methods on BitVector::Iterator with
STL-compatible operator overloads, and add begin/end methods to
BitVector itself, so that BitVectors can be iterated with ranged for
loops.

As a drive-by cleanup, make GrowableBitVector hold the BitVector by
value (to avoid needing to allocate one for empty iteration), and remove
its unused (and inefficient) Union method.

Change-Id: Idcd34e26bfb087e3ec8297b4a769a51bfab4b6e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3455803Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79071}