This reverts commit 5df9ae3f.

Also updates other references based on Preview 1 and moves from
Microsoft.Composition to System.Composition

* Update toolset compilers to 2.3.0-beta2-61716-09. Use LangVersion=7.1
* Revert LangVersion for VB. Follow-up issue #19610

Now that we are using the correct version of IBCMerge we can re-enable its usage on our
workspaces components.

https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?id=297536&_a=edit&triage=true

Re-enable roslyn analyzers on Roslyn.sln and update public api file for an IOperation API change merged yesterday (#19278).

* Update versions

* Improve diagnostics

The former always has the meaning we want, the latter has it only
when there are no Returns used anywhere in the same file!

This both removes our flaky dependency on a file share and sets us up to
get updates for the package.

The VS SDK team provided us with an updated NuGet that has better error
diagnostics during install / uninstall.  Going to change our VSI tests
to begin using build based deployment again for VSIX until they can get
the diagnostics that they need.  Once they have them will flip it back.

The VS SDK team is interested in tracking down the deployment errors
that we are seeing when deploying as a part of build.  This change adds
an option to make it easy to excercise that code path in builds.

Going to use this option to quickly toggle to the build based deployment
in order to mass test their fixes.

The Net20 framework projects were accidentally depending on 3.5
framework in order to build.  This is due to how they generate
resources.

By default when generating resources for a 2.0 framework project the
MSBuild task from 3.5 was being used.  This is not installed by default
with Visual Studio and only through the 3.5 framework.

This went unnoticed when moving to the new SDK because all of the
Jenkins machines and @jaredpar machines have it installed.

Setting the `$(GenerateResourceMSBuildRuntime)` property to
CurrentRuntime will use the GenerateResource task from the current
version of MSBuild.  This is how resources were being generated before
the migration.

This was reverted as a part of 5fe4adb3 but should have been
put back.

The Microbuild leg now almost completely builds.  It's failing in the
validation of our compilers Nuget package.  This comes from subtle
changes to Toolset.csproj that need to be addressed.

This does represent a huge step forward though.  Saving this now so I
can experiment with that change.

The content produced by NuGet restore is independent of build
configuration.  Hence it needs to be stored in a folder that is
accessible to both Debug and Release.

This didn't work with the present structure of our Obj directory
because it put project folders under configuration directories.  That
mean there is no common parent for a project across all configurations.
To fix this we need to change our Obj structure:

- Old: `Obj\<Configuration>\<Project Name>`
- New: `Obj\<Project Name>\<Configuration>`