This change drops support for .NET 4.5 and retargets to desktop
4.6 + CoreCLR. It also fixes up various analyzer warnings that
were created due to the move.

Fixes #12926

The dependency projects serve a few purposes:

- Help ensure package unity by serving as a single place for a NuGet reference.
- Make NuGet updates simple by having a single place to change.
- Grouping together related packages to make it easy to take a dependency on say Visual Studio editor.

The first two points are largely obsolete now that our project.json references are strictly validated on every build and have a single update mechanism.  This meant a lot of our depnedency projects were just making Roslyn.sln bigger.

As such I went through and deleted all of them which had only 1 or 2 project.json entries.  I left the VS ones which tend to group 10+ references together.  They are still serving a valid "reference VS" purpose.

These were workarounds for building with Visual Studio 2015 RTM. We
don't support that anymore.

Fixes #8767.

This reverts commit 31377ce7.

This reverts commit 7debfc47.

Update TargetFrameworkVersions.

To prevent projects across the tree from having different dependencies, we unify
external package dependencies by creating empty C# projects that represent them.
We then make use of project.json package inheritance (where a project inherits
all packages dependencies from their dependencies) to unify it across the tree.

No projects outside of this tree, should be directly referencing versions of
these dependencies manually.

For this check-in I've tackled Metadata, Reflection, and DiaSymReader. Will tackle
the rest in future commits.

This reverts commit b87b2712.

This reverts commit 662593ba.

This changes the Mac builds to use coreclr for running csc and vbc

With Visual Studio 2015 Update 1 you should not specify -anycpu
in the runtimes section. Right now we'll have both to allow us to build
with and without Update 1, and we'll remove the old ones once we've
made the transition.

Update 1 also tries deploying implementations for some class libraries,
so I'm updating all our test and VSIX deployment class libraries to
now include implementations.