[Polly][Isl] Use isl::union_map::unite() instead of isl::union_map::add_map(). 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:
 - Use `isl::union_map::unite()` instead of `isl::union_map::add_map()`
 - `isl-noexceptions.h` has been generated by this https://github.com/patacca/isl/commit/3f43ae29fa2a22936a583b85b2fe8d439f805d8d

Depends on D106059

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D106061