* Move Roslyn to newer toolset packages

1. Move to 1.2.1 compiler toolset package.
2. Move to 1.2.0-beta1 analyzer packages.
3. Turn on IOperation feature flag for all projects as the new analyzer packages have IOperation based analyzers.

* Use the Roslyn wide default CA rulesets for newly added samples project.

* Fix couple of test projects to use the Roslyn wide rulesets for code analysis.

* Move to analyzers 1.2.0-beta2 packages (multiple fixes and enhancements to Roslyn diagnostic analyzers)

* Revert accidental file mode changes.

* Move shipped implicit constructors to public API shipped files.

Fixes #11446

This reverts commit 31377ce7.

Update TargetFrameworkVersions.

the comparison needs to be more than a simple `==` to handle retargeted
assemblies

we only need to care about CompilationReference since other metadata reference we explicitly make sure we use same reference unless it is changed.

added unit test for reference equality

New types "WordSimilarityChecker" and "SpellChecker" now use these types to provide
higher level operations that can add more heuristics into the mix (which are
unrelated to the inner operations of EditDistance and BKTree).

We don't need SolutionDir or RestorePackages, which were for very old-
style restore systems.

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* This removes all references to NuGet.targets (which is obsolete and no longer exists)

Moving simplifier tests from ServiceTest to EditorServicesTest2

Move TestUtilities.csproj and TestUtilities.FX45.csproj to separate directories to accomodate current nuget limitations

These were causing warnings in IDE due to #4192 and must be
been added by a bad package.

The targets files were previously split into an open version in
Microsoft.CodeAnalysis.Toolset.Open and a closed version in build/. This
unifies the four targets into two new targets, VSL.Settings.targets and
VSL.Imports.targets, and moves all the targets from ..Toolset.Open into
the build/ folder.

If we have a solution with circular references, leave one as a metadata
reference to prevent deadlocks when trying to get compilations.

The `FilePathUtilities.GetRelativePath(...)` method is used in various
places in the Workspaces and VS layers to generate a relative path from
a starting directory to a target file or directory. However, it has a
bug where it will generate an incorrect path if you need to go up more
than one level from the starting directory to get to the target.

For example, if the starting directory is `C:\Alpha\Beta\Gamma` and the
target is `C:\Alpha\SomeFile.txt`, the correct relative path would be
`..\..\SomeFile.txt`. Instead, we get `..\..\..\SomeFile.txt` (note the
extra `..`).

This commit fixes the issue and adds unit tests for this method.

This deletes the empty and unused Desktop assemblies from the code base.

This removes the majority of references to the deprecated Desktop
assemblies.  The only remaining ones are in the VSIX projects.  They
must remain until we actually delete the assemblies.

Upgrades System.Collections.Immutable to 1.1.34-rc and
System.Reflection.Metadata to 1.0.19-rc.