Deadlock in cancelled build

_Originally reported as https://developercommunity.visualstudio.com/content/problem/377146/vs2017-hangs-when-building-a-project.html. Dump analysis by @tmeschter._

It's possible to deadlock MSBuild when the main thread has a lock on `BuildManager._syncLock`, and is waiting for the logging event queue to finish up:

```
[0x23] mscorlib_ni!System.Threading.Tasks.Task.Wait(Int32, System.Threading.CancellationToken) + 0x35 ​
[0x24] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.TerminateLoggingEventQueue() + 0x45 ​
[0x25] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.WaitForThreadToProcessEvents() + 0x19 ​
[0x26] Microsoft_Build_ni!Microsoft.Build.Execution.BuildSubmission.CompleteLogging(Boolean) + 0x3a ​
[0x27] Microsoft_Build_ni!Microsoft.Build.Execution.BuildManager.ExecuteSubmission(Microsoft.Build.Execution.BuildSubmission, Boolean) + 0x4c0 ​
[0x28] Microsoft_Build_ni!Microsoft.Build.Execution.BuildSubmission.ExecuteAsync(Microsoft.Build.Execution.BuildSubmissionCompleteCallback, System.Object, Boolean) + 0x4f ​
```

```
[0xc] clr!JITutil_MonReliableContention + 0x8a ​
[0xd] Microsoft_Build_ni!Microsoft.Build.Execution.BuildManager.OnProjectFinished(System.Object, Microsoft.Build.Framework.ProjectFinishedEventArgs) + 0x3e ​
[0xe] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.TryRaiseProjectFinishedEvent(Microsoft.Build.Framework.BuildEventArgs) + 0x2e ​
[0xf] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.RouteBuildEvent(System.Collections.Generic.KeyValuePair`2) + 0x4a ​
[0x10] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.RouteBuildEvent(System.Object) + 0x154 ​
[0x11] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.LoggingEventProcessor(System.Object) + 0x85 ​
[0x12] Microsoft_Build_ni!Microsoft.Build.BackEnd.Logging.LoggingService.b__115_0(System.Object) + 0xa ​
```

This codepath will only be traveled if `_shuttingDown` (that is, the build has been cancelled):

https://github.com/Microsoft/msbuild/blob/0f8106a6ef9b6347e43815cc842130d2ed349ef5/src/Build/BackEnd/BuildManager/BuildManager.cs#L929-L937

Otherwise, work is posted to the work queue, but the lock is released before executing it.