ExpressionVariableFinder should properly handle misplaces DeclarationExpressions with ParenthesizedVariableDesignation.

Fixes #17572.
This change also ensures consistent SemanticModel behavior between valid and invalid usages of DeclarationExpressions.

* Add breaking changes doc for post-VS2017
Fixes #17089 

* Make the compiler resilient to broken IDE refactorings
* Restore incorrect assertions that happen not to be triggered by existing tests.

This makes the compiler resilient to bad syntax trees that result from broken
IDE refactorings, so the compiler will not crash and take the IDE with it.
Related to #17321

FIxes:#17395

NOTE: binding of `ref readonly` parameters is NYI

* Have distinct LocalDeclarationKind for OutVariable, DeconstructionVariable
Fixes #13898

Fixes #12052

Correclty handle local functions.

Fixes:#17140

Fixes #16935.

Fixes #16919.

* Add tests to improve coverage of pattern-matching implementation and fix a crash.
Fixes #9542
Fixes #16876 (crash that is fixed)
Fixes #17088
Added tests for #17089 

Fixes #8018.

Fixes:#16947

Even for cases when the parameter is not part of a type member declaration, i.e. owned by an anonymous method or a local function.
Fixes #16748. Fixes #16374.

Also implement SemanticModel.GetDeclaredSymbol(ParameterSyntax) for parameters of a Local Function.
Fixes # 16757.

The subsumption tree must treat an unconstrained type parameter as possibly null.
Also, if a previous case pattern handles all non-null input, any further case type pattern is subsumed.
Fixes #16688
Fixes #16671

Fixes #16559

Attributes will not be permitted on type parameters or parameters of
local functions. This matches the existing behavior for the return type
and symbol itself, neither of which allow attributes to be applied to
them.

Fixes #1640

Fixes #13206.

Fixes:#14600
Fixes:#11013
Fixes:#14116
Fixes:#16168

Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=336030
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=294570
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=363714
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=336030
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=364165
Fixes #16296
Also adds test for #16559

Fixes #16478.

Fixes #16517 

Extensions methods declared on bases and implemented interfaces are allowed to be applied to receivers that of derived or implementing type.

This change extends this rule to tuple receivers as long as the rule holds for individual elements, recursively.

I.E. valid "this" conversions are:
- identity conversion
- implicit reference conversion
- boxing conversion
- (new)  implicit tuple[literal] conversions as long as all underlying conversions are valid "this" conversions.