[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

> 1 Is size_t appropriate here? I see some functions using unsigned, and some using size_t. 

I think it's an appropriate type to use (`unsigned` would also be fine, but I tend to prefer `size_t` for this sort of use, personally).

> 2 On this new flow to get a string literal, there's at least 2 allocations of the string from the 2 cxstring::createDup, one when evaluating the result, and one when returning it. Is this ok/can it be done better?

I was thinking about that and I think it's okay. We do not have a public `createDup()` interface for `CXString`, which means returning a `CXString` with the same lifetime as the eval result can be a bit tricky because the user has to do their own buffer management if they want the string to outlive the eval results. If we had such a public interface, then I would think it's better to not have the second duplicate and leave it to the caller to decide the lifetime questions.

>  3 Is returning a "null" CXString on error a good idea?

I think so.

I've added a few more reviewers for some broader opinions.

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