Ignore return value of signal() on Android

[marcprux]

Fixes #3180 by ignoring return value of signal() on Android

[finagolfin]

My Android CI just hit this too, and this pull fixes it when I tried it natively on Android. I don't know why simply defining the type and a temp variable for the output fixes this error, but it does.

[glbrntt]

Thanks!

[marcprux]

I don't know why simply defining the type and a temp variable for the output fixes this error, but it does.

I'm guessing that nullability annotations are making the return value non-nil for Android, but nullable on Darwin.

func f0() -> (() -> ())? {
    { }
}

f0() // warning: Result of call to 'f0()' is unused

func f1() -> (() -> ()) {
    { }
}

f1() // error: Function is unused
_ = f1() // fine

[glbrntt]

@marcprux could you rebase your branch on main? GitHub isn't giving me the option for some reason...