[Polly] Refactoring IsInnermostParallel() in ISL to take the C++ wrapper object. NFC

Currently, the IslAst library is a C library that would be incompatible with the rest of the LLVM because LLVM is written in C++.
I took one function, IsInnermostParallel(), and refactored it so that it would take the C++ wrapper object instead of using reference counters with the C ISL library. As well, all the references that use IsInnermostParallel() will use manage_copy() since they are still expecting the C object.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D97425