Hard-to-reproduce GC bug

[dsimcha]

I'm having issues where it appears that the D2 druntime GC ignores roots
located in thread-local storage when determining what objects are alive.  This
leads to spurious deletion of objects in certain cases.  This is clearly a GC
issue, since disabling the GC solves it, but I can't figure out how to
reproduce it in a canonical way.  Has anyone else run into anything like this?