[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Damian Andrei via cfe-commits]

xTachyon wrote:

That's pretty much what I was thinking of in terms of implementation.

> and we'd need the versioning information if we had Clang N tracking length + contents and Clang N + 1 was tracking length + contents + encoding because the newer Clang would be casting to a pointer of the wrong type if it was given a CXString from the older Clang.
>
> But now that I think about it, this still runs into ABI issues like the original solution. Newer Clang would produce a private_flags value that older Clang couldn't handle gracefully.

Does libclang support having different versions of it at the same time loaded in a process? Otherwise I don't see how the outside world would be able to tell or care what `data` and `private_flags` mean.

In my suggestion, `StringWithLength` would only have a definition inside `CXString.cpp`; I don't want to make it part of the public API. I believe this would mean that this can be changed at any point for any reason.

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