Calling Yield() without Reacquire() then returning from a task hangs MSBuild

Here's a simple, buggy task embedded in a project:

```xml

```

Note that `Reacquire` is commented out--but the task completes successfully.

MSBuild just hangs when this happens

```sh-session
❯ msbuild .\yield.proj
Microsoft (R) Build Engine version 16.9.0-preview-20630-02+a2c42ce9f for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

Build started 1/5/2021 2:41:15 PM.
```

And if you ctrl-c,

```
Attempting to cancel the build...

Unhandled Exception: Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: One or more errors occurred.
=============
System.AggregateException: One or more errors occurred. ---> Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: One or more errors occurred.
=============
System.AggregateException: One or more errors occurred. ---> Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: MSB0001: Internal MSBuild Error: Entry must be active before it can be Completed successfully. Config: 2 State: Waiting
=============
Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: Entry must be active before it can be Completed successfully. Config: 2 State: Waiting
at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Object[] args)
at Microsoft.Build.Shared.ErrorUtilities.VerifyThrow(Boolean condition, String unformattedMessage, Object arg0, Object arg1)
at Microsoft.Build.BackEnd.BuildRequestEntry.Complete(BuildResult result)
at Microsoft.Build.BackEnd.RequestBuilder.ReportResultAndCleanUp(BuildResult result)
at Microsoft.Build.BackEnd.RequestBuilder.d__50.MoveNext()
```

This is not catastrophic because it's a task-authoring error: you're supposed to `Yield()` and `Reacquire()` in matched sets. But it sure is awkward, when we could either

1. Implicitly reacquire when `ITask.Execute()` returns, or
2. Replace the manual yield/reacquire business with a `using`-compatible resource.