[libunwind] libunwind: Implement the unw_strerror function for better nongnu libunwind compatibility (PR #160887)

[Gleb Popov via cfe-commits]

arrowd wrote:

> As far as I can tell (and Gleb, please give explicit confirmation as to whether this is 100% accurate or not), no code has been copied

That's right and it is pretty obvious from the diff.

> the variable names and formatting are different, it uses early return rather than variable assignment, and this even has the default case after the end of the switch rather than inside it

It also has a LLVM-specific `UNW_ECROSSRASIGNING` case, which is absent in nongnu libunwind.

> only the English text used for most of the error messages (possibly all but the arm64 PAC one?).

Yes, I copied string constants from nongnu libunwind, mainly because I'm unsure if there is software in the wild that depends on them. Of course, I don't mind reformulating them.

https://github.com/llvm/llvm-project/pull/160887