CppTools: Tag incomplete semantic info

...in order to be able to full-rehighlight on the next turn.

The following sequence was problematic:

1. recalculateSemanticInfoDetached(true)
   * e.g. triggered by opening the document
2. recalculateSemanticInfoDetached(false)
   * e.g. triggered by moving the cursor
   * cancels 1. and leads to incompletely parsed/checked document - OK
3. startHighlighting()
   * triggered by 1.; starts highlighting on incomplete document - OK
4. startHighlighting()
   * gets a completely parsed/checked document - OK
   * not forced, so just compare revisions; they are the same, so
     skip/return - a partly highlighted document is left behind.

Task-number: QTCREATORBUG-11367
Change-Id: Ic56e00e862ec4a1ffa197b2fc8b48be56a3562de
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>