CppTools: Add basic test for TypeHierarchyBuilder

Change-Id: I7a82f2b22105d4e096fc0bd69679adacdde2c731
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppTools: Speed up TypeHierarchyBuilder::buildDerived()

Depending files of the file the base class is defined in were look up
for *all* the derived classes. With this change, only the relevant files
for the 'current' base class are looked up.

Tested with qtbase.pro via Ctrl+T on the QObject definition.
    - Without this patch: ~60s
    - With this patch: ~40s

Change-Id: Ia947f1a4f7d242a0cb81e42d0ef2afab2db70a7f
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
CppEditor: Reset completion view before aborting

...otherwise subsequent events (e.g. Return) might trigger functions
operating on an invalid model index (see activateCurrentProposalItem).

Task-number: QTCREATORBUG-10589

Change-Id: Ib3e37c8c85c882135970d5d9418d6e69917494c6
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>