Modifying TestRunner to support both Legacy (v1.9.2) and Upgraded (v2.1+) xUnit.

Stop hiding exceptions on continuation tasks

Fix binary expression formatting

 1. Wrap every async method in an exception filter that reports a
 non-fatal Watson, prints and error message, and then propagates the
 exception.  We do this so that dumps are taken before exceptions are
 stashed in tasks and the stack is unwound.

 2. Wrap the command filter in an exception filter that reports a
 non-fatal Watson, suppresses the error, and forwards to the next filter
 in the chain.  Propagated exceptions would just turn into useless
 dialogs.

 Fixes #4553

Exceptions in (or under) ```FinishExecute``` were being lost.  As a
result, some of our tests were passing incorrectly.

Instead of trying to manage everything ourselves using ```ContinueWith```
use ```async``` and let the system propagate exceptions for us.

Fix attributes on ReplCommandCompletionProvider...

This is intended to prevent this provider from being loaded in *normal*
C# files.

Fixes #4769

Enable horizontal scrolling in Interactive Window

Make BreakLine follow the cursor

Stop reporting unused usings in submissions

Previous replaced spans should not expand

Fixes #4755

Support multi-dimensional arrays of modified types.

Properly ref count diagnostic taggers.

Disable checks for non-compliant overloading for extension methods to match VS2013 behavior

Since they can be used in subsequent submissions, they are always
considered to be used.

Fixes #4392

Fix Native View tests on x64

Pointer width was hardcoded - switched to using ```PointerToString```.

Bonus: Fix cast exception in attempt to report the failure.
Bonus: Make Native View hex lowercase to match the rest of the
ResultProvider.

Don't include `public` accessiblity modifier in completion inside an interface

Escape System.Delegate when it is committed without qualification

Fixes #4405 

Copy current line if no selection

Fix #732. C# completion list must not suggest `public` as a potential
accessibility modifier to use inside an `interface { }` block;
otherwise, it produces code that does not compile.

Remove unnecessary call to AsImmutableOrEmpty