Currently, namespace_decls have no record of how deep they reside in the 
namespace tree.  That makes is_ancestor overly complex (and therefore 
bleeds over into unqualified namespace lookup)

This patch adds a SCOPE_DEPTH macro (it may be useful to extend to other 
scope-like things).  It uses the base.u.bits.address_space field, which 
is available to us.

We 'only' permit 255 levels of namespace.

nathan

-- 
Nathan Sidwell