Dedup import scopes based on value equality instead of reference equality

Fix race in MultipleSimultaneousCompiles

made populate initial tag to filter out stale ones outside of given t…

Several instances of the RunCompilationAsync helper methods are spawned off in this test and utilize the shared resource `Temp`.  If these end up resuming in parallel they can race on calls to `List<T>.Add` and throw an exception. Fix the issue by making the method `static`, and doing the shared data management in the non-async versions of the code.

closes #9630

Use processes to parallelize our big unit test assemblies

Merge stabilization into master - 1

Merge remote-tracking branch 'upstream/stabilization' into merge-stabilization-into-master20160309-160047

Merge stabilization into master

Correct build order dependency for VB result provider

Fixed race in diagnostic tagger

This project uses the VB syntax generator hence needs a reference to the project in order to guarantee it's built first.

closes #7869

Couple of items:

- Plumb AssemblyInfo through the code base
- Remove most usages of ResultsFileName from caching layer

Now factors in the maximum windows command line length.  It also has
improved logging.

Turn off hard-linking during the build

We turned on hard-linking to reduce copy and build time, however,
this would result in behavior where copying a NuGet binary from .nuget/packages to the output directly which was then subsequently overwritten with a different version would result in the .nuget\packages folder to be corrupted because they were now linked.

Move to RC toolset

more allocation reduction.

This reverts commit b03e8478.

This changes the toolset we use to build Roslyn on our developer boxes to 1.2.0-rc.  Very close to release, need to make sure we're eating the latest brand of dogfood 😄