Create MSBuild properties to make it easier to find and consume RoslynTools.Microsoft.VSIXExpInstaller.

We're going to need this soon, so may as well start restoring it now to make it easier to test.

Our references were previously a mix of preview 4 and some build between preview 4 and 5.  That was causing build output double writes.

Updates the compiler used in the Linux and Mac toolsets and also
eliminates the requirement to download the toolset as a zip and instead
restores the necessary components over NuGet.

Putting the GIT SHA into unofficial binaries was hurting our efforts around determinism.  It meant that binaries authored by developers which had the same code, but against different HEAD values, would end up with distinct binaries.  Hence anything in our infrastructure which leveraged determinism for caching wasn't getting the benefits it should.

The primary use of the GIT SHA is to help us track back from a released binary to the source that created it.  Especially important given the frequency with which we produce NuGet packages.  Hence it only needs to be present on official built binaries as they are the only ones we release.  Having it into the developer builds doesn't have any notable value.

Our script for downloading NuGet.exe now ensures the local copy is the correct version.  Previously it assumed any local copy was the correct version.  Lead to problems when we brought down a new version that had bug fixes necessary for building the repo.

Portable unit tests are difficult because as portable libraries they can't truly be built in a runnable fashion.  Instead they need a deployment project to provide a runtime on which to delpoy the output of the build.  To support running our portable tests on both desktop and CoreClr the build is structured as follows:

- Portable tests build w/o deploy to a Dll directory
- Desktop delpoyment references all portable tests and outputs to UnitTests\Desktop\Portable
- CoreClr deployment references all portable tests and outputs to CoreClrTests

This breaks F5 for the moment but gives us the flexibility to restore later.

Ensure that we set the OptimizationDataFile path after importing the core targets, which sets $(TargetName) that is used by this property.

are no Dev15 instances installed on the machine.