libclang: refactor CXStringPool: make it a class

We are not exposing the pool or string buffers to libclang users, so no need to
maintain a procedural interface.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173595 91177308-0d34-0410-b5e6-96231b3b80d8