Completion does not work in the Immediate Window after expression evaluation

[ivanbasov]

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/809625
Fixes https://developercommunity.visualstudio.com/content/problem/478452/intellisense-in-immediate-window-does-not-work-sec.html

[jasonmalinowski]

@ivanbasov Did we also want to do the cleanup of:

1. Instead of having RemoveContext() call Dispose(), have it have the full logic for cleaning up and disposing (i.e. just inline the Dispose() contents)
2. Dispose() can call RemoveContext() of course to remove anything that's left, but then that's an explicit case.

As @AmadeusW observed, Dispose() is really intended to be "final", and it's odd that we're using that as a random helper in a most-definitely-not final action.

[ivanbasov]

Thank you, @jasonmalinowski ! I have swapped Dispose and RemoveContext in DebuggerIntelliSenseFilter. However, AbstractDebuggerIntelliSenseContext.Dispose still has a wide scope. I'm not sure we we should introduce one more method here and put everything from Dispose there.