Revert "[Polly][Isl] Removing nullptr constructor from C++ bindings. NFC."

This reverts commit be5e2fc7bf781c7fc079943552ea1b519f45c815.

This introduced a building error for polly. https://lab.llvm.org/buildbot#builders/10/builds/4951

GitOrigin-RevId: f60ea691a94b3b4c28f2ff85a4520562f4110810
13 files changed