Replace repository version of ISL by 'make dist' output

The 'make dist' archive is not dependent on ./configure output and
contains a GIT_HEAD_ID file that identifies the version of ISL used.

None of the files added or removed are used part of Polly's build
process (except of GIT_HEAD_ID since the previous revision r240301). No
functional change intended.

llvm-svn: 240306