[Support] Compile fix for gcc. NFC.

gcc 5.4 insists on template specialization to be in a namespace polly { ... }
block, instead of being prefixed with 'polly::'. Error message:

root/src/llvm/tools/polly/lib/Support/GICHelper.cpp:203:54: error: specialization of ‘template<class T> void polly::IslPtr<T>::dump() const’ in different namespace [-fpermissive]
   template <> void polly::IslPtr<isl_##TYPE>::dump() const {                   \
                                                      ^
msvc14 and clang 3.8 did not complain.

llvm-svn: 282874