Ports the compiler generator tools (syntax generator, bound tree
generator, et al.) to .NET core and runs the exes on CoreCLR instead of
the desktop framework.

Fixes #7136

This reverts commit 760e1204.

Fixes - #8951

This reverts commit e2fc5135.

Synchronize all corefx references to -23504 and bring in the
xunit coreclr test runner.

Visual Studio keeps wanting to modify the solution,
letting it, and checking that in.

This is old detritus, and didn't actually build. But if you accidentally
built from a x64 build window, it'd break you.

Moves the build task code to a shared project that we use to build
at net45 and a portable50 CodeAnalysis MSBuild task. Also upgrades to
nuget v3.2.1-rc since this is necessary to understand the dotnet4.3 spec
used in the project.json.

- rename it to Roslyn.Compilers.Extension so it's clearer what it is
- remove the old specification of binding redirects. Now that Update 1
  provides us with the ability to patch the language services with
  a proper VSIX, we don't need to redirect anything other than the build
  task anymore.
- update the set of deployed binaries to the current compiler set
- clean up package code and registration

to indicate that the timestamp isn't a real timestamp
Fixes #5940

They are no longer needed.  Time to remove them.

The recommendation tests can be run in parallel but were in an assembly which
had many other tests that could not. Splitting out into a separate assembly
which is designed for parallel runs.

This change makes foreground scheduling code predictable in the IDE
tests.  Instead of using a static to hold the data we use instance
values.  The static data the instance values are populated from are not
100% predictable but far more so than they used to be.

* This updates all references to nuget.exe to use the copy in the root.
* This deletes all NuGet.config and NuGet.exe files that are not in the root.
* This removes all references to NuGet.targets (which is obsolete and no longer exists)

Rogjt mpw we build a boostrap version of csc and vbc and bootstrap
with those running on Mono. This commit switches us to build a CoreCLR
compatible csc and then running the boostrap on CoreCLR

Right now the full set of Linux CoreCLR runtime assemblies is not
available on NuGet, so we substitute with some assemblies we stash in
our NuGet ZIP package.