[Polly] Use isl::set::tuple_dim instead of isl::set::dim. NFC

This is part of an effort to reduce the differences between the custom C++ bindings used right now by polly in `lib/External/isl/include/isl/isl-noxceptions.h` and the official isl C++ interface.

Note that not all the usages of `isl::set::dim` were replaced