[isl++] abort() on assertion violation.

Before this patch, ISL_ASSERT only printed an error message to stderr.
This can be easily missed if the program continues or just fails later.
To fail-early and help error diagnostics (e.g. using bugpoint), call
abort() when an assertion does not hold.

I seem to just have forgotten to add this abort() when I originally
proposed the ISL_ASSERT macro.

Suggested-By: Eli Friedman <efriedma@codeaurora.org>

Differential Revision: https://reviews.llvm.org/D45171

llvm-svn: 330467