Cloog: Add virtual destructor to silence gcc warning

This silences the following warning:

  'polly::ClastVisitor' has virtual functions but non-virtual destructor
  [-Wnon-virtual-dtor]

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@177798 91177308-0d34-0410-b5e6-96231b3b80d8