CppEditor: Fix override lookup for double call and casting

Possibly fixes other use-cases

Task-number: QTCREATORBUG-11159
Task-number: QTCREATORBUG-11165

Change-Id: I375143c9658c581267c7c8d61ce03c27858f2de5
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>