[Support] Add (Nonowning-)IslPtr::dump(). NFC.

The dump() methods can be called from a debugger instead of e.g.

    isl_*_dump(Var.Obj)

where Var is a variable of type IslPtr/NonowningIslPtr. To ensure that the
existence of the function pointers do not depdend on whether the methods are
used somwhere, they are declared with external linkage.

llvm-svn: 282870