* Add missing facades to devdiv toolset

Fixes http://ddweb/buildstatus/issues/issues.aspx?iid=819217

This extends the behavior to constructors, destructors, and operators.
Fixes #17257

Make Fix-All use normal text-diffing as the TreeDiffer produces some very bad results.

Porting C# Classification Tests

* use compare exchange for the my template

* switch Object.Equals to Is

Fixes #17548

Generate 'null' for optional nullable values that are equal to 'null'.

Handle MissingPredefinedMember in AsyncRewriter

Support 'ValueTasks' in the 'make method async' fixer.
Fixes #17470

Add SemanticModel tests for ObjectCreationExpressionSyntax in yield return statement.

Do not run editor commands if designer file is opened be default

Make local function default parameter value binding lazy

Current strict binding can cause circularity problems when local
functions are referenced. This change causes local functions to use lazy
default parameter binding, similar to methods, and then forces their
construction when diagnostics are requested for the local function.

This also requires a mechanism for recording declaration diagnostics
outside of adding to the compilation's DeclarationDiagnostics. A new
type, DeclarationDiagnosticStore is introduced as an abstraction to
store declaration diagnostics on either the compilation or in a local
DiagnosticBag, depending on the needs of the symbol storing diagnostics.

Fixes #16451, #17293

Closes #3825.

Merge dev15.1.x to master

Add a new Azure Function-based merge tool

Support 'Clearing' of the FAR windows if a language requests it.
Fixes #17475

Make passing of CompletionItemRules more explicit.

NullReferenceException in CheckFeatureAvailability with `out _` and SpeculativeSemanticModel

Ensure LocalRewriter doesn’t crash when it has to deal with ErrorMethodSymbol.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems?id=377890.

* add ubuntu 16.04 to CI

* added ubuntu.16.04 to the list of runtimes

* make ID unique

Add remaining ChangeSignature Integration Tests