Drop Cloog support

  This commit drops the Cloog support for Polly. The scripts and
  documentation are changed to only use isl as prerequisity. In the code
  all Cloog specific parts have been removed and all relevant tests have
  been ported to the isl backend when it was created.


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@223141 91177308-0d34-0410-b5e6-96231b3b80d8
Make checkout isl script executable

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@223146 91177308-0d34-0410-b5e6-96231b3b80d8