Override completion broken for Razor documents with code behind

[davidwengier]

Version Used: Version 17.14.0 Preview 2.0

Steps to Reproduce:

1. Create a new Blazor Web App
2. Open Counter.razor
3. Navigate to the @code block and run Extract to code behind
4. Go back to Counter.razor
5. Add an empty @code { } block.
6. Inside the block, type override so override completion comes up
7. Observe yellow bar

Stack trace:

System.InvalidOperationException : Unexpected null - file AbstractMemberInsertingCompletionProvider.cs line 162
   at Microsoft.CodeAnalysis.Contract.Fail(String message,Int32 lineNumber,String filePath)
   at Microsoft.CodeAnalysis.Contract.ThrowIfNull[T](Nullable`1 value,Int32 lineNumber,String filePath)
   at Microsoft.CodeAnalysis.Completion.Providers.AbstractMemberInsertingCompletionProvider.ComputeDestinationSpan(SyntaxNode insertionRoot)
   at async Microsoft.CodeAnalysis.Completion.Providers.AbstractMemberInsertingCompletionProvider.RemoveDestinationNodeAsync(<Unknown Parameters>)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at async Microsoft.CodeAnalysis.Completion.Providers.AbstractMemberInsertingCompletionProvider.DetermineNewDocumentAsync(<Unknown Parameters>)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at async Microsoft.CodeAnalysis.Completion.Providers.AbstractMemberInsertingCompletionProvider.GetChangeAsync(<Unknown Parameters>)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at async Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformFunctionAsync[T](<Unknown Parameters>)

From a quick debug it seems like its trying to generated the override method into the code behind file. Perhaps fall out from #76821

/cc @CyrusNajmabadi @arunchndr @phil-allen-msft as this is probably 17.14 worthy

[davidwengier]

Also seeing other weirdness with override completion in general. In a razor file with a lot of C# code in a @code block, I can bring up override completion at the top of the code block, and when accepting it, the implementation ends up 500 lines lower down in the file. Seems like maybe something has lost a "if completion then context position is really important" check.