recursive-patterns(3): Implement the decision DAG and subsumption checking.

Improve handling of array builder in in LocalRewriter.LowerIsPattern

recursive-patterns(2): Add support for recursive patterns in `is`

Fixes #22969

Size array builders at creation time.
Eliminate more uses of `var` in place of explicit types.
Use Hash.Combine

Replace "TODO(patterns2)" with "PROTOTYPE(patterns2)"

Merge features/compiler into features/recursive-patterns

Merge master to features/compiler

We now forbid the use of `var` as a pattern's type except in the var pattern, where it is forbidden to bind to a user-declared type. Requires LDM and compat council review.

Related to #14592

Move us to SDK 2.2.0 preview

Remove background compiler from TestWorkspace

without additional disambiguating syntax, per 2017-11-20 LDM decision.

Avoid allocations in IsDefinedInSourceTree hot paths