Hard-to-reproduce GC bug

[dsimcha]

== Quote from Sean Kelly (sean at invisibleduck.org)'s article
> Weird.  The actual storage for TLS in druntime is an array of void*
> within the Thread class.  I can't imagine that it wouldn't be scanned by
> the GC.  Do you have a reproducible test case?
> Sean

I just now managed to play around with this some more and come up with a small
test case, as opposed to a much larger real-world case, that reproduces this.  I
still haven't the slightest clue *why* my latest test case reproduces the bug and
some others that I had tried didn't, but I've filed a bug report.  See:

http://d.puremagic.com/issues/show_bug.cgi?id=2491