[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Damian Andrei via cfe-commits]

xTachyon wrote:

> What if we add unsigned clang_getCStringLength(CXString); which gets the length but not the contents? Do we need to package pointer and size together?

Where do we store the length, if we can't add a new field to `CXString`, and we can't use `private_flags` to store it?

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