[analyzer] Mark `lookupPromotedThisType` method as experimental.

This API method is part of an unreleased and unfinished feature
(`this-promotion`). Mark it as experimental so that customers aren't
tempted to rely on its behavior until it's ready to ship.

(I should have done this when I first introduced the
method. Fortunately, we haven't released the analyzer since I
introduced it, so it won't break anyone to mark it as experimental
now.)

Part of https://github.com/dart-lang/sdk/issues/63816.

Change-Id: I3aea9d45c9cca64d2e8fe0bc9104b7726a6a6964
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/543620
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Auto-Submit: Paul Berry <paulberry@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>