[Polly][Isl] Use isl::*::ctx instead of isl::*::get_ctx. NFC

[Polly][Isl] Use isl::*::ctx instead of isl::*::get_ctx. 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::*::ctx()` instead of `isl::*::get_ctx()` (for example `isl::space::ctx()` instead of `isl::space::get_ctx()`)
 - Add `isl::` namespace in front of isl types to avoid confusion (for example `isl::space::ctx` and `isl::ctx`
 - `isl-noexceptions.h` has been generated by this https://github.com/patacca/isl/commit/b64e33c62d3fe7db506f6630f9e935b663f9c5a4

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D105691