[_fe_analyzer_shared] Compute multiple witnesses

This updates the exhaustive checking algorithm to produce a list of
witnesses instead of a single witness in case of a non-exhaustive error.

This is used to create a witness for each missing subtype when checking
"sealed static types", for instance enums and sealed classes.

The current error reporting isn't changed and only reports the first
witness. Use of the multiple witnesses in error messages and/or lints
must be done in the CFE and analyzer.

Change-Id: I950816f6a9eca16773f182d5d820929bdcb39684
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/357160
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>