Switch some .ConfigureAwait(true) to .ConfigureAwait(false)

* Ensure modreq must exist on PE symbols that C# compiler expects them to have

* PR feedback

* More tests + feedback

* Skip tests on mono due to bug

* Added Assert.True(IsOverride) for overrides

Enable TestPreviewDiagnosticTaggerInPreviewPane

These tasks are being waited for with a .WaitAndGetResult(); using
.ConfigureAwait(true) means if they don't run synchronously then
we will schedule something to the UI thread while simultaneously
blocking the thread. That will deadlock the product, and also tests.
Turns out the latter happens on a semi-regular basis.

Fixes dotnet/roslyn#21563.

Fixing a typo

Fix the Fast Up-To-Date check for Roslyn.sln

Merge dev15.6.x to dev15.7.x

Fixes #14444

Directory.Build.props and Directory.Build.targets are handled by the
project system, so are not included in this change.

we should remove these once partner moved to new API.

Fix VerifyRenameTrackingWorksAfterInlineRenameCommit

Fixes #15225

Merge dev15.6.x to dev15.7.x

* removed dead code

* added a way to order Document/ProjectDiagnosticAnalyzer.

this is to let partner team such as typescript and F# to be able to order their analyzers

* use => instead of {get;} =

* PR feedbacks

Notify the remote workspace of working folder paths in the background

Add cast to prevent the dynamic binder from doing the cast

* Improve error tolerance when parsing invalid implicit arrays.

Provide helper for common pattern.

Update test.

* Be resilient to infinite loops.

IVsRunningDocumentTable4.GetDocumentData() returns dynamic, and we
were casting it to an interface. This was going through the C#
runtime binder to do so. We don't need that, and an unrelated change in
VS meant that Microsoft.CSharp.dll isn't ngened, so this was far more
expensive than it should be.

@olegtk and @sharwell get credit for the fix.
Co-authored-by: NOleg Tkachenko <olegtk@microsoft.com>
Co-authored-by: NSam Harwell <sam.harwell@microsoft.com>

Merge dev15.6.x to dev15.7.x

Add Xunit adapter to enable testing in VS

* fixed a bug on solution crawler progress report.

found case where retry can mess up Start/Stop call. instead trying being smart to reduce Start/Stop call, made it simpler and rely on ref counting on reporter. and the fact that all enqeued work must run at some point.

* updated comment

* renamed and added comments

Produce an error when a bridge method must be variadic

Merge dev15.6.x to dev15.7.x

This is necessary to install pre-release .debs of the Mono runtime.