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

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.

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

Create the Packages.props and FixedPackages.props.  These are the files
which will contain our MSBuild mapping from packag name to package
version