[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I think this problem is an important one to solve, and this is not the first time our rigid ABI has caused us problems. I wonder at what point we should decide to break ABI, because there are other cleanups we also want to do (removing deprecated APIs, for example).

I've posted https://github.com/llvm/llvm-project/pull/141657 in an attempt to make it more clear that we can make some changes here even though the `private_flags` value may change in a way that's ABI breaking.

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