Fix building with non-Visual Studio 2015

Merge stabilization into master

Reenable the CurrentWorkingDirectory test

We conditionally add this in a target to work around Visual Studio's
poor handling of conditions on Reference nodes in the project system.

In future versions of Shell.14.0, some types have been type forwarded
to this assembly. This means we'll take a reference to Immutable.14.0
when building with 14.0, but that's fine since we won't use it.

Add version variables for Esent, Elfie and InteractiveWindow

Revert "Revert "Merge pull request #6636 from CyrusNajmabadi/asyncLazy""

Fix unit test deadlocks

Fix the MSBuild warning check

More refactorings in Perf Test tool

There are runs on Jenkins where the IDE is failing the assertions of running on the foreground / background thread.  This causes an exception to be emitted and generally fails the resulting test.  In the case of this bug though the exception was causing the test to deadlock due to this particular call.

http://source.roslyn.io/#Microsoft.CodeAnalysis.EditorFeatures/Implementation/Classification/SyntacticClassificationTaggerProvider.TagComputer.cs,203

``` csharp
_notificationService.RegisterNotification(() =>
    {
        _workQueue.AssertIsForeground();
        ReportChangedSpan(snapshot.GetFullSpan());
    },
    ReportChangeDelayInMilliseconds,
    _listener.BeginAsyncOperation("ReportEntireFileChanged"),
    _reportChangeCancellationSource.Token);
```

Here the callback throws due to the failed foreground check.  The implementation of RegisterNotification wasn't properly hardened to ensure `IAsyncToken` was disposed in the face of an exception.  This caused that particular token to never complete and deadlocked the unit tests.

closes #7825

Work around MSBuild race condition

Enc: async/iterator editing regression fix

Make compiler VSIX extension aware of VS versions

Prepare cibuild.cmd to receive /buildTimeLimit

The compiler was hard coding the version of MSBuild to be 14.0.  This works on Dev14 but fails in Dev15 where MSBuild is 15.0.  This changes the extension to pick the MSBuild version based on the current version of Visual Studio.

Misc Perf cleanup

If we don't have this, CreatePkgDef is getting angry. My guess is the
assembly is no longer in the GAC, and so it's being implicitly relied on
that the assembly is copied by MSBuild for CreatePkgDef to find it.

This is just so it's not the only project in our entire source tree
doing something different.

For the Shell binary, we have two choices. We could either reference the
version that matches the version of Visual Studio you are building with,
or we could reference a fixed version with a HintPath so we can find the
binary on all versions of Visual Studio. Doing the former is tricky
since the name of the assembly is embedded into some XAML, so I go with
the latter option.

This is the implementation assembly with implementation details that
we should not depend upon from our shipping binaries.

This has been set for all releases we care about, so we can delete this.

This doesn't hardcode an assumption on 14.0.

PIAs always have the version of the Visual Studio they shipped with,
even in higher versions of Visual Studio. We should reference the fixed
version that matches the PIA.

We use some public types from assemblies that aren't in the SDK. We
previously picked those up from the GAC but there's no guarantee that
they are there.