Merge dev16.0-preview3-vs-deps to master-vs-deps

Merge remote-tracking branch 'upstream/dev16.0-preview3-vs-deps' into merges/dev16.0-preview3-vs-deps-to-master-vs-deps

Merge dev16.0-preview3 to dev16.0-preview3-vs-deps

Merge dev16.0-preview3-vs-deps to master-vs-deps

Re-enable mono CI build

Merge master to master-vs-deps

Disable Mono CI build

Use Visual Studio 2019 for integration testing

Updated dotnet-format install instructions

Do not offer to remove redundant assignment if it is not parented by …

Fix documentation tests

- Added instructions for installing from NuGet.org

Move MSBuild installation directory to Current

Require dev16 for integration testing

The reason for this should be clear in the change.

Fix up support for changing rule set files through the Solution Explorer 

* Fix naming, remove help button, let the UI has a default selected destination

* Add comments

* remote question mark from warning dialog

Merge master to master-vs-deps

Merge dev16.0-preview3 to master

Merge dev16.0-preview3 to dev16.0-preview3-vs-deps

Merge master to master-vs-deps

Remove unnecessary test hooks

Disable integration tests with <95% reliability

Bail out for error types from IDE0058 analyzer (unused expression value)

MSBuild has changed their installation versioning story. Components now
insert into a directory named Current instead of a directory name
matching the Visual Studio major version (i.e. 15.0). This changes
Roslyn to match the new design.

Note: this change must be inserted into VS in coordination with MSBuild.
Otherwise the insertion will fail.

https://github.com/Microsoft/msbuild/issues/4069
https://github.com/Microsoft/msbuild/issues/3778

The AnalyzersCommandHandler was still trying to call GetHostProject
to find the host project as a way to directly grab the rule set file
to process that. That was deprecated but we hadn't fixed it up yet.

The fix is to re-thread through a way to get to the rule set. The
"ownership" of a rule set file (if one exists for a project) is owned
by the VisualStudioProjectOptionsProcessor, so we now need a way to jump
to that directly from any project so another map is added to the
VisualStudioWorkspaceImpl. That map is unfortunate -- the
OptionsProcessor is really something that should be private to the
project system itself and shouldn't be grabbable by any random code.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/779916
Completes task https://devdiv.visualstudio.com/DevDiv/_workitems/edit/698029

* Enable LinkedFileDiffMergingEditorTests.TestCodeActionPreviewAndApply test

* Enable InteractivePasteCommandhandlerTests.PasteCommandWithOutInteractiveFormat test

* Updated LinkedFileDiffMergingEditorTests with PR feedback
Co-Authored-By: NJoeRobich <joseph.robichaud@microsoft.com>