Related to #12597.

Related to #12597.

Fixes #12883

Also update the out var syntax node to use the newly introduced
VariableComponent so it can be extended in the future to allow `out *`
or `out var (x, y)` etc without requiring any further changes to the
syntax model.
Fixes #12747, #12664, #12588, #1503

  (e.g. ref local variable declaration)
We introduce a "ref expression" for use in expressions
  (e.g. right-hand-side of a ref variable initialization)
Also improve the diagnostic for local functions.
Fixes #12749, #12580

[Based on features/async-return with tests ported from that branch.]

Fixes: 11282
Fixes: 11875

- Implicit tuple type conversion exists between tuples of same cardinality when all corresponding element types have implicit conversions
- Implicit tuple type conversion is a standard implicit conversion and therefore can chain with user defined conversions
- Implicit tuple type conversion is a standard conversion that can be lifted to nullable

Also addressed feeback provided for the previous change.

- Feature flag.
- Syntax model changes.
- Parsing.
- Binding (for explicitly typed variables only).
- Flow analysis.
- SemanticModel – added new API GetDeclaredSymbol(ArgumentSyntax) to return symbol for the declared local.

Basic scenarios work end to end.

Remove unnecessary allocations and layers of translation.
Related to #8817.

Add an instrumentation component that can be used to instrument certain statements and expressions during lowering. Part 2.

This finalizes the refactoring. Sequence points are no longer created before LocalRewriter is invoked. LocalRewriter completely relies on instrumentation for sequence points generation. There is an exception, LocalRewriter creates hidden sequence points in three places, but they are not associated with any executable code derived from C# syntax.

Related to #9819.

Related to #8817.

Related to #8817.

Related to #8817, #8814.

Also implement more robust long term fix for #9612.
Related to #8817.

Fixes #9624.