Fixes #12900

* Implement source link feature

* Fix CommonCompiler to open source link stream, add E2E tests

* Reduce code duplication across VB and C#

  (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

For the most part, this change implements the same encoding
for tuples that we use for dynamic. This means that interface
implementations and generic constraints are not covered, but
base types and type parameters are.

This change also doesn't have full tests for "long" tuples (nested tuple
transformations) or tuple types that can only be constructed in
metadata.

This implements lowering and good code generation for typeswitch. However, if the switch can be
handled using the old strategy, we completely reuse the old binding and lowering. That enables
us to not lose edit-and-continue for existing switches.

The bound switch label now contains the value, as the labels cannot be relied on to contain the
constant values (e.g. synthesized labels do not, and we now synthesize switches for typeswitch).
Many places that work with bound switch statements had to be adjusted to consistently use the
constant in the bound switch label.

Fix parsing bug where `case A when B:` was parsed as a type pattern `A when` because `when` is
an identifier, followed by a syntax error (`B` seen but `:` expected). The fix is to never let a
pattern variable be named `where` at the top level of a switch.

Related to #12316

Fixes: 11282
Fixes: 11875

Also, added tests for Out Variables Declarations within attribute applications.

Also addressed feeback provided for the previous change.

Implements subsumption checking for pattern-matching.
Fixes #8823, #10487, #10601, #10632

- 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.

Since "original" is by itself contextual on the presence of "replace", only "replace" actually needs to be keyed off a flag.

Tuples: lookup well-known ValueTuple members relative to given underlying type (not well-known type)