[Polly][Isl] Refactoring IslAstInfo::getBuild() and...

[Polly][Isl] Refactoring IslAstInfo::getBuild() and IslAstInfo::IslAstUserPayload::Build to use isl++. NFC

Polly uses 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 method `IslAstInfo::getBuild()`
 - `IslAstInfo::IslAstUserPayload.Build` now uses C++ types instead of C types
 - Removing destructor of `IslAstInfo::IslAstUserPayload`

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D104370