C++: fix scope matching for templates in FindUsages

When the cursor is on the name of declaration of a templated function,
then since f93758b8e1d68ed28dc1a84b71c906d58784cb70 the scope returned
by Document::findScopeAt is the scope of the template declaration.
Before it was the parent scope of the template declaration.

The check in FindUsages::checkCandidates did not check all combinations
of template(-child symbol) scopes for the searched symbol and its
occurrences.

Task-number: QTCREATORBUG-9749

Change-Id: Idc84a2ba718721ce54683a67635a93352784ddd1
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>