[Polly][Isl] Stop generating isl::union_{set,map} from isl::space. 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 generating `isl::union_set` and isl::union_map` from `isl::space` and instead generate them from `isl::ctx`
 - Disable clang-format on `isl-noexceptions.h`
 - Removed `isl::union_{set,map}` generator from `isl::space` from `isl-noexceptions.h`
 - `isl-noexceptions.h` has been generated by this https://github.com/patacca/isl/commit/87c3413b6f1d62ca3dddf716352f90a0b8533353

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D106059