[Polly] Refactoring astScheduleDimIsParallel to take the C++ wrapper object. NFC

Polly currently needs to be slowly refactor to use the C++ wrapper objects to handle the reference counters automatically.
I took the function of astScheduleDimIsParallel and refactored it so that it uses the C++ wrapper function as much as possible.

There are some problems with the IsParallel since it expects the C objects, so the C++ wrapper functions must be .release() and .get() first before they are able to be used with IsParallel.

When checking the ReductionDependencies Parallelism with the Build's Schedule, I opted to keep the union map as a C object rather than a C++ object. Eventually, changes will need to be made to IsParallel to refactor it to the C++ wrappers. When this is done, this function will also need to be slightly refactored to not use the C object.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D98455