Updating the xplat toolset solution to match the refactored test utility project names.

Remove dependency on particular evaluator implementation

Make Interactive window command parsing case sensitive.

Add encoding detection for CoreCLR

```InteractiveWindow``` tacitly assumed that
```IInteractiveEvaluator.ResetAsync``` would a newline-terminated string
to the current language buffer, making it safe to insert a new prompt at
the cursor position.  Obviously, this behavior is implementation-specific
and cannot be relied upon.

Instead, store the uncommitted input and remove the current language
buffer.  If the evaluator adds input, then a new one will be created for
it.  Otherwise, things will be tidy when the window creates a new prompt
and restores the uncommitted input.

Fixes #4518

Update to System.Reflection.Metadata alpha-00012

On CoreCLR we will always be unable to fetch the default encoding and
the 1252 encoding will be unavailable unless we reference
System.Text.Encoding.CodePages. However, we don't want to take a
dependency on this assembly on desktop 4.5 so we now use
AssemblyLoadContext to try to detect whether or not we are running on
CoreCLR and, if so, reflection load the necessary assemblies to get hold
of the necessary types.

Improve ResetCommand

Remove unnecessary if statement

Fix #4121,  #4125 and #4277 

Associate PropertyComment with Target on AttributeListSyntax

Fix that completion list doesn't include param entries for indexers

Restructure and rename Roslyn.Test.Utilities projects

Fix NRE in command line parsing

Allow await in declarations in C# Interactive

Fixes #4418

 1. Unflip the noconfig argument (fixes #4396).
 2. Classify ```noconfig``` anywhere in the argument list.

More test refactorings

Split the public API files

Express command visibility in vsct...

This will allow VS to figure out when to display the "Reset Interactive from Project" command without having to load the Interactive assemblies (execute code).

Move portable sources from TestUtils to TestUtils2

Fix xml doc gen with escaped identifiers

The command line parser was inconsistent in how it set the
sourceFilesSpecified boolean flag.  In one place it tracked whether the
source files collection was non empty and in another it tracked whether
or not we had attempted to parse an argument as a source file.  This
disconnect lead to a NRE when there was an invalid source file and the
command line tried to calculate the output name for a library project.
Changed the compiler to consistently treat it as tracking whether or not
a valid source file was specified.

This change splits the files for public API tracking into:

- PublicApi.Shipped.txt: APIs shipped in the previous version of Roslyn
- PublicApi.Unshipped.txt: APIs shipped in the next version of Roslyn

This makes it very easy for us to tell at a glance the API change which is being
carried in a given branch of Roslyn.

Don't bother creating taggers when there are no diagnostics to display.

Enable InteractiveConsole.BreakLine