Rebase C++ bindings on top of latest isl bindings

The main difference in this change is that isl_stat is now always
checked by default. As we elminiated most used of isl_stat, thanks to
Philip Pfaffe's implementation of foreach, only a small set of changes
is needed.

This change does not include the following recent changes to isl's C++
bindings:

  - stricter error handling for isl_bool
  - dropping of the isl::namespace qualifiers

The former requires a larger patch in Polly and consequently should go
through a patch-review. The latter will be applied in the next commit to
keep this commit free from noise.

We also still apply a couple of other changes on top of the official isl
bindings. This delta is expected to shrink over time.

llvm-svn: 338504