IslNodeBuilder: Add virtual function to obtain the schedule of an ast node

Not all users of our IslNodeBuilder will attach scheduling information to the
AST in the same way IslAstInfo is doing it today. By going through a virtual
function when extracting the schedule of an AST node other users can provide
their own functions for extract scheduling information in case they attach
scheduling information in a different way to the AST nodes.

No functional change for Polly itself intended.

llvm-svn: 247126