[Swift-devel] hang checker updates

[Mihael Hategan]

I think mike requested swift stack traces in the hang checker instead of
cryptic thread ids. That's in now.

Also in is a dependency loop detector in the hang checker. It doesn't
detect static cycles, but ones that actually cause a hang. I'm not sure
how well it works for real life situations, but I can confirm it works
for simple things like a = f(b); b = f(a);. Please give it a shot.