IslExprBuilder: Provide PointerLikeTypeTraits for isl_id

Providing an explicit PointerLikeTypeTraits implementation became necessary
since LLVM started in https://llvm.org/svn/llvm-project/llvm/trunk@256620
to automatically derive the pointer alignment from the pointer element type,
which does not work for incomplete types as used by isl. To ensure our code
still compiles, we provide an instantiation of PointerLikeTypeTraits for isl_id
which assumes no minimal alignment. isl pointers are likely to have a "higher"
alignment. We can exploit this later in case this becomes performance relevant.

llvm-svn: 256650