[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Damian Andrei via cfe-commits]

xTachyon wrote:

> It means all such string accesses would require two indirections to get to the string data

I don't think that's true. Getting the pointer would be `ptr+sizeof(size_t)`.

> I also wonder if we want to be slightly less memory efficient and add a version field in case we need to make changes again in the future

Where would this go? The `Data` structure would be private API, not exposed, so it shouldn't matter when we change it.

I don't understand very well the last messages of you and [cor3ntin](https://github.com/cor3ntin), sorry.

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