Clang: Request diagnostics/highlighting only if needed

Most of the time, the updated diagnostics/highlightings will be send by
the backend on translation unit update.

The other use case is changing the font settings (e.g. color), here we
need to request the highlightings explicitly.

Change-Id: I17a574eaf972c8bef12900241e7b33fe6ffd9dbd
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>