[analyzer] Plug "temporary leak" when starting new rebuilds before `afterContextsCreated` is called

If again and again editing, say, a `pubspec.yaml` with proper timing
there is a "temporary leak" that repairs itself once the analyzer
finishes (some time after the editing stops).

What happens is that old contexts are saved in the
`declarationsTracker` but eventually cleared in the
`afterContextsCreated` call.

In a test on Windows (on Linux I'd currently run into
https://dartbug.com/52703) I opened the entire "pkg" folder and edited
a `pubspec.yaml` file every 5 seconds.
The analyzer went from using something along the lines of 700MB of heap
to using around 2.5 GB of heap after 25 edits and 17GB (!) of heap
shortly before stopping at 250 `pubspec.yaml` edits.
After the editing stopped (and clicking the GC button in observatory)
the heap usage went down to ~650MB heap used.

This fixes the problem by clearing the `declarationsTracker` on the
`afterContextsDestroyed` call too. In the same test it stays at around
300-700MB of heap.

Possibly related to:
https://github.com/dart-lang/sdk/issues/48788
https://github.com/dart-lang/sdk/issues/52447

Change-Id: Ia38cc946a1f36fa8c5b6804f79cbc8dd96c21030
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/309722
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>