Associate module EnC diagnostics with changed documents

[tmat]

Also fixes the following issue:

1. Changes are made in closed files (e.g. branch change)
2. Attach to process
3. Workspace receives file watcher notifications for files changed in [1]
4. Attempt to step is blocked because we think there are changes in the solution but the runtime does not allow making changes

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1463707

[tmat]

@davidwengier