Fix error closing source link documents in VSCode

[dibarbet]

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2222267/ where closing a sourcelink document would crash the server.

There were two changes that led to this issue

1. When I implemented Adds support for metadata LSP language features in VSCode #74488, I implemented it on the assumption that IMetadataAsSourceFileService.TryAddDocumentToWorkspace and IMetadataAsSourceFileService.TryRemoveDocumentFromWorkspace actually added and removed documents from the workspace - which was true for metadata and decompilation. This meant if the document was already found in a workspace, we would skip calling TryAddDocumentToWorkspace in LSP.
2. When I implemented Add support in DevKit for source link go to definition #74626, I failed to realize that PdbSourceDocumentMetadataAsSourceFileProvider.TryAddDocumentToWorkspace and PdbSourceDocumentMetadataAsSourceFileProvider.TryRemoveDocumentFromWorkspace did not actually add/remove from the workspace - that happened in GetGeneratedFileAsync.

So what happened was that TryAddDocumentToWorkspace was sometimes not called because the sourcelink document was already added to to the MetadataAsSource workspace when the original go to definition request was made. So closing the document would throw because the document was never opened.

There were a couple ways to fix this. I chose modifying PdbSourceDocumentMetadataAsSourceFileProvider to match the contract to add and remove the document from the WS in TryAddDocumentToWorkspace TryRemoveDocumentFromWorkspace.