Update to isl-0.17.1-314-g3106e8d

This version includes an update for imath (isl-0.17.1-49-g2f1c129). It fixes
the compilation under windows, which does not know ssize_t.

In addition, isl-0.17.1-288-g0500299 changed the way isl_test finds the source
directory. It now generates a file isl_srcdir.c at configure-time, containing
the source path, to not require setting the environment variable "srcdir" at
test-time. The cmake build system had to be modified to also generate that file.

llvm-svn: 288811