(PUP-1279) Make Class[::p] remove the ::

When transformed to 3x to form a reference to a class, the 3x runtime
does not handle ::p (it expects the :: to have been removed), and
subsequently it is not able to find classes starting with ::.

This fix removes leading '::' when a PHostClassType is created.
Merge pull request #2446 from hlindberg/pup-1979_class-ref-with-absolute-name

(PUP-1279) Make Class[::p] remove the ::