Use intrusive refcounted pointers to manage RopeRefCountString lifetime.

std::shared_ptr<char []> would be even nicer, but shared_ptr doesn't work
with arrays :(

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217798 91177308-0d34-0410-b5e6-96231b3b80d8