[clang] [libclang] Always Dup in createRef(StringRef) (PR #125020)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The public libclang interface needs to be stable, yes... but in this context, that's just the interfaces declared in clang/include/clang-c/CXString.h. We can do whatever we want with the internal interface in clang/tools/libclang/CXString.h.

https://github.com/llvm/llvm-project/pull/125020