CppEditor: Accept LookupContext in virtual function lookup

Required for correct resolving of first virtual appearance

Change-Id: I2307027f769fb2f4c0942f4aa4e0d2b5327562b5
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
CppEditor: Return first virtual function on is[Pure]VirtualFunction

Change-Id: I086076cc58c78430e025a78703a028610024ed23
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
CppEditor: Add failing tests for virtual destructor lookup

Change-Id: I2fdf1c72b3e5ffe25b5184c1161a803c4427945b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>