[llvm] [Support] Re-raise external signals (PR #125854)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Makes sense. I don't know why SIGQUIT in on the Kill list instead of Int list. Nevertheless, Kill/Int are handled similarly. If an external process raises SIGSEGV/SIGQUIT, the default action is termination. With the handler, the process should terminate after removing temporary files and invoking some handlers.

It's strange that this
```
if (Info->si_pid != getpid())
    raise(Sig);
```

pattern isn't that common.

(@emaste)

https://github.com/llvm/llvm-project/pull/125854