[clang] clang_EvalResult_getAsCXString impl (PR #134551)

[Damian Andrei via cfe-commits]

xTachyon wrote:

`CXString::private_flags` only has 2 bits used. Could we use the rest for the size? Would it be enough?

There's also the possibility to not touch `CXString` at all, and add a function that returns ptr+size only for `clang_EvalResult_getAs...`.

What do you think?

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