[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> What are the ABI guarantees of libclang? Won't adding a new field to CXString break compatibility?

That's a great point, it would be an ABI breaking change and we don't want those in libclang: https://github.com/llvm/llvm-project/blob/25e08c0b9cafaab09af35ce6a03317ffd503df08/clang/include/clang-c/Index.h#L33

https://github.com/llvm/llvm-project/pull/134551