[llvm] [LLVM][Cygwin] Fix Signals compatibility with Cygwin API (PR #138117)

[via llvm-commits]

jeremyd2019 wrote:

Cygwin's error path here (uninitialized buffer) is probably not any worse than Linux's (uninitialized pointer).  After this is merged I can look at dealing with it (it'd likely conflict).  The easiest thing would be to `continue` on error, but it might be better to output a placeholder name of `(error)` or something like that instead.

https://github.com/llvm/llvm-project/pull/138117