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

* 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* This removes all references to NuGet.targets (which is obsolete and no longer exists)

MSBuild currently produces a warning for any target framework that
doesn't have a directory containing its reference assemblies, but
DNXCore50 purposfully has no reference assemblies and currently has no
empty directory to indicate so. Issue #5213 details figuring out what
the proper solution is here, but in the meantime this silences the
warning by pretending that the TargetFrameworkDirectory for DNXCore50 is
.NETPortable, 5.0.

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.

This is part of the fix of issue#4774.

This change does two actions to our public API files:

- Resets them to the versions from 1.0.0 RTM
- Splits them into the shipped and unshipped files

We no longer use SCC bindings in our project yet the definitions remained around and caused a dialog prompt on solution open.  Removing these as they are no longer needed

This only takes effect if you're building in a versionless command
prompt.

Rather than failing the build, if Roslyn is installed globally just
don't try to deploy.

Fixes #4189 (https://github.com/dotnet/roslyn/issues/4189)

If we don't include this, we lose some build machine validation,
including anything that processes VS SDK specific files like
.vsixmanifests, .pkgdefs, or .vsct files.

It looks like MSBuild has a bug on *nix where they aggressively
change directory separators from '\' to '/', even when the '\'
is being used as an escape character in a define constant in VB.
This change works around the bug by removing all quotes from
define constants after Microsoft.VisualBasic.CurrentVersion adds
them in our build. This should be OK since none of our constants
should require quoting.

This fixes the Build/AssemblyVersion to be the same as RoslynSemanticVersion when OfficialBuild is true, but no BuildVersion was passed in.

This fixed DevDiv.1158790 — Spurious errors, warnings and messages in IDE from background analysis, which
basically results in a bunch of warnings in the IDE:

     The referenced component 'C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETPortable\v4.5\Profile\Profile7\mscorlib.dll' could not be found.

C#/VB project system do not support conditional references and treat them as always being present. This <Reference was being included for all projects regardless
of the condition, confusing Visual Basic project system. Moved it to _ExplictReference concept which already exists for adding mscorlib (see <_ExplicitReference> in Microsoft.CSharp.CurrentVersionCommon.targets)

InternalsVisibleToTypeScript items are showing up the Solution Explorer visual tree
as missing items (because the C# project system treats them as files). Hide them
like the other InternalsVisibleToXX items.

This changes the 'StartProgram' used by our Unit Tests to be the same as used by our CI Builds.