C++: Do not set binding block as instantiationOrigin

The binding in a block should not be used as an instantiationOrigin,
because it will be deleted if it does not contain any blocks or
namespaces or classes or enums.
Instead of that we should look for first enclosing class or namespace.

Task-number: QTCREATORBUG-11424
Change-Id: I0fc6e935495478f71372b0fe6f611887f45b2eda
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>