[Polly] Refactoring isInnermost() from isl to use the C++ wrapper

Polly use algorithms from the Integer Set Library (isl), which is a library written in C and which is incompatible with the rest of the LLVM as it is written in C++.

Changes made:
 - Refactoring isInnermost() to take C++ bindings instead of the plain isl C api.
 - Addition of manage_copy() when needed to get the reference for the isl_ast_node object

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D99841