recursive-patterns(4): Parsing the switch expression.

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

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

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

Nullable reference types in C# Proposal Link

Move assert outside loop

Need to be on 15.4 in order for the .NET core workload to bring along
the 2.0.0 runtime.

The .NET Core workload brings down the shared runtime component. That is
needed when our core clr tests are run.