[Polly] Partially refactoring of IslAstInfo and IslNodeBuilder to use isl++. NFC.

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 the following methods of class IslAstInfo
   - isParallel() isExecutedInParallel() isReductionParallel() getSchedule() getMinimalDependenceDistance() getBrokenReductions()
 - Refactoring the following methods of class IslNodeBuilder
   - getReferencesInSubtree() getScheduleForAstNode()
 - Refactoring function getBrokenReductionsStr()
 - Fixed the mismatching function declaration for getScheduleForAstNode()

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D99971