Naming the execution scripts 'csc.sh' and 'vbc.sh' is counter to the
convention of not using any extension on *nix executables. While the
scripts are not strictly executable programs, they are meant to be
transparent shims around the actual execution of the compilers on *nix

After updating to the latest CoreCLR NuGet packages we no longer need to
be copying binaries from a drop share to run. This change adds a csc.sh
and a vbc.sh so we can run on the CoreRun host on Linux like we do on
Windows. The change also eliminates some cruft from the cibuild like the
semaphore restore.

Update to a new version of the Roslyn Diagnostics analyzer package.

The -f option doesn't include files that don't already exist, while the
-u option updates existing files and also adds files that don't exist.

Also, we were deleting the zip file after unzipping, leading the caching
to always fail and download new zip files every time, so this should
allow caching to work again.

Right now we parallelize by sticking multiple mac builds on the same machine. Since NuGet and Mono toolset restores are machine-global, sometimes the builds stomp on one another. This adds a semaphore to only allow one build to restore at a time.

Updates the mono toolset to mono built from source at tag
mono-4.2.0.207.

Updates the mono toolset to mono built from source at tag 4.2.0-207.

The native compiler handled corrupted default parameter values by
substituting in default(T).  It's unclear if this was intentional
behavior in the compiler or not.

Either way though obfuscators took advantage of this behavior and at
least one prominent one will corrupt default parameter values when the
value is null.  Enough prominent libraries have shipped using such
obfuscators that it is a blocker to upgrading.  Hence we need to emulate
the native compiler behavior here.

This change is a bit large because I needed to update the test resources
with a corrupted DLL in order to test out the changes.

closes #4196

The cross platform legs are causing roughly ~10% of all our Jenkins jobs to fail
(3% Mac failure, 7% Linux failure).  The main source of the instability is the
NuGet and build step.  As a short term work around I'm adding retry logic to
those steps to get the false failure rate down.

This reverts commit 4ca8634b.

Occasionally the mono installs on the Jenkins machines get corrupted and
needs to be cleaned out.  Added an option that will not use the cached
install and instead force download it from Azure blob storage.

It's confusing for new users to have the Roslyn solutions nested
inside the src/ directory and there's really no good reason those where
there in the first place. This moves them to the root directory, except
for the Toolset.sln solution, which is only for building Roslyn in CI
scenarios and has been moved to the build/ directory.

Now that Mac and Linux are using the same version of Mono, we can extend
the Linux support to match that of Mac.

The cibuild scripts had diverged between stabilization and master. This
change brings stabilization back in sync with master and makes a small
change to ensure passing invalid commandline arguments returns a proper
error code.

This changes our cross plat build to install the desired version of Mono
on the machine as a part of the CI process.  This lowers the management
overhead of our Mac / Linux machines as we can use any machine in the pool
vs. having reserved, and custom maintained, machines for Roslyn.