This CL refactors and extends the infrastructure around sending
diagnostic notifications. This enables publishing lint errors as
warnings after a compilation run.

R=sigurds@chromium.org, tebbi@chromium.org

Bug: v8:8880
Change-Id: Ia64d2d490c1449021c92f5dc45eb7f8dab21e60a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1582405
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61003}