[analyzer,cfe] Support contained types in as-pattern

This adds a new rule to exhaustiveness handling of the as-pattern. The
new rule checks whether the cast type is fully contained in the space
of the subpattern. If so, we can assume that the whole context type has
been matched because there is no value that passes the as-test that
would be rejected by the subpattern.

Closes #51986
Closes #54125

Change-Id: I0dc7fb072395aa3bc9f0b143afb320966f9d64c5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338760
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>