Revert "[polly] [ScopInfo] Don't pre-compute the name of the Scop's region."

This reverts commit 0f9dc03765dc301fff7a52e2a0e1dd3e5f3130c5, r328666.

The change introduced a use-after-free, caused by the temporary name string
being destroyed after converting it to a StringRef.

llvm-svn: 331363