Convert the pimpled copy-on-write SvGlobalName class to use the
::o3tl::cow_wrapper using the default reference counting policy.

Change-Id: I7bceb06ddfb31ca5901e5e7d5d93dda494db945f
Reviewed-on: https://gerrit.libreoffice.org/18070Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>