[Polly][Isl] Stop using isl::set::lex_le_set. 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.

Changes made:
 - Stop using `isl::set::lex_le_set`. The official way to do this is to use `isl::map::lex_le_at`
 - Removed `isl::set::lex_le_set` from `isl-noexceptions.h`
 - `isl-noexceptions.h` has been generated by this https://github.com/patacca/isl/commit/266fea1d3dbd31c23d866eb363fcc8e61e50419f

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D106269