Don't crash graph checking on invalid syntax

[0101]

Another case similar to #16575

Ends up throwing in VS with Transparent Compiler as you're typing.

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cc @dotnet/fsharp-team-msft

[majocha]

Another surfacing exception:

Just after enabling it in options.

[0101]

Just after enabling it in options.

That must have to do with the fact that it needs a restart to actually start using it. But some of the options are read on every file check so there must be some mismatch. Should hopefully be easy to fix.

[0101]

The test failure here also looks suspicious, most likely a bug.

The active test run was aborted. Reason: Test host process crashed : Unhandled exception. System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at System.Threading.CancellationTokenSource.Cancel()
   at FSharp.Compiler.GraphChecking.GraphProcessing.raiseExn@233[a](CancellationTokenSource localCts, TaskCompletionSource`1 completionSignal, a item, Exception ex) in /home/vsts/work/1/s/src/Compiler/Driver/GraphChecking/GraphProcessing.fs:line 234
   at FSharp.Compiler.GraphChecking.GraphProcessing.queueNode@253-7.Invoke(FSharpChoice`2 res) in /home/vsts/work/1/s/src/Compiler/Driver/GraphChecking/GraphProcessing.fs:line 253
   at Microsoft.FSharp.Control.AsyncPrimitives.CallThenInvokeNoHijackCheck[a,b](AsyncActivation`1 ctxt, b result1, FSharpFunc`2 userCode) in /home/vsts/work/1/s/src/FSharp.Core/async.fs:line 528
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in /home/vsts/work/1/s/src/FSharp.Core/async.fs:line 112
--- End of stack trace from previous location ---
   at Microsoft.FSharp.Control.AsyncPrimitives.Start@1173-1.Invoke(ExceptionDispatchInfo edi) in /home/vsts/work/1/s/src/FSharp.Core/async.fs:line 1173
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in /home/vsts/work/1/s/src/FSharp.Core/async.fs:line 112
   at <StartupCode$FSharp-Core>.$Async.clo@193-15.Invoke(Object o) in /home/vsts/work/1/s/src/FSharp.Core/async.fs:line 195
   at System.Threading.QueueUserWorkItemCallback.Execute()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart()

[psfinaki]

Sorry I crashed your merge but otherwise good stuff - thanks :)