After changing language dropdown, diagnostics are not updated to reflect the new language

[brettfo]

REPRO:

1. Create a new C# notebook.
2. Set the cell contents to [1;2;3;4] |> List.sum
3. Wait for the C# diagnostics to appear; 'Identifier expected'.
4. Change the cell language to F#.

EXPECTED: The diagnostics should disappear because they're no longer valid.

RESULT: The stale C# diagnostics are still present until the cell is changed by typing.