[Polly][Ast] Partial refactoring of IslAst and IslAstInfo 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 `IslAst`
  - `getAst()` `getRunCondition()` `buildRunCondition()`
  - Removed the destructor in favor of the default one
 - Change the type of the attribute `IslAst.RunCondition` to `isl::ast_expr`
 - Change the type of the attribute `IslAst.Root` to `isl::ast_node`
 - Change the order of attributes in class `IslAst` to reflect the data dependencies so that the destructor won't complain
 - Refactoring the following methods of class `IslAstInfo`
  - `getAst()` `getRunCondition()`

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D100265