Enable expression variables in constructor initializers and expand their scope to constructor body.

Merge 'dotnet/features/compiler' into ExpressionVariables

Merge dev15.7.x to features/compiler

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

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.

Merge dev15.7.x to features/compiler

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.

LOC CHECKIN | roslyn dev15.6.x | 20180202

Merge dev15.7.x to features/compiler

Fixes:#24348

Merge dev15.5.x to dev15.6.x