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.

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

Add a when clause to the switch expression's cases
Add support for binding the switch expression (but not decision tree)
Implement flow analysis for switch expression
Adjust precedence of the constant pattern in a switch expression arm.

- Move `PartialCaseDecision` inside `DecisionDagBuilder`.
- The implementation of equality for `BoundDagEvaluation` and `BoundDagTemp` are moved to separate source files.
- `SyntheticBoundNodeFactory.AssignmentExpression(LocalSymbol left, BoundExpression right)` is removed as unused.
- Removed commented-out code in `LocalRewriter_Patterns.cs` and `PatternParsingTests.cs`

Simplify parsing a subpattern list.
Check recursive pattern feature at the top level of the pattern only.

by using out parameters instead of assigning to locals in the enclosing scope.

- [x] https://github.com/dotnet/roslyn/pull/23208#discussion_r151519706 try-finally can be eliminated in local function `looksLikeCast` in `ParseExpressionOrPattern`
- [x] Recommend adding some blank lines back to Syntax.xml. See https://github.com/dotnet/roslyn/pull/23208#discussion_r151802628 and https://github.com/dotnet/roslyn/pull/23208#discussion_r151802657 .
- [x] Parameter unused in `private ScanTypeFlags ScanType(bool forPattern = false)`; See https://github.com/dotnet/roslyn/pull/23208#discussion_r151807815 . Also the same comment for the caller; see https://github.com/dotnet/roslyn/pull/23208#discussion_r151808973 .
- [x] Prefer types spelled out instead of `out var` (including in `LanguageParser.cs` and `LanguageParser_Patterns.cs`); See https://github.com/dotnet/roslyn/pull/23208#discussion_r151808453 and https://github.com/dotnet/roslyn/pull/23208#discussion_r151808516 et al.

- Remove BoundDagEvaluation.Symbol from the public API
- BoundDagTemp is not permitted a null type
- Adjust test for new lowering of pattern-is
- Adjust tests per changes in `is pattern` code generation
- Lower RecursivePattern.VariableAccess
- Retire or comment out (for now) some retiring code.
- Move MatchConstantValue into the class with its only client.
- Use the more efficient equality for double, decimal, etc switches.
- Implement and test support for tuples.
- Implement building the decision tree and its subsumption errors.

- Add support for recursive patterns in `is`.
- Add the discard pattern `_`.
- Add skeletal IOperation support.
- Adjust tests per changes in `is pattern` code gen.

Add tests to make sure virtual slot is set correctly for parent types of types that have modreq members or implementations (#22646)

* Add tests to make sure virtual slot is set correctly for parent types of types that have modreq members or implementations

* PR comments

* Compiler API not to return [Obsolete] for IsByRefLikeTypes

* Clean up

* typo

Fixes #20103

* fixed using block

* updated tests after synching master

* PR feedbacks

Merged on behalf of @svick. Thanks for the contribution!