If the backtracer crashes, at present we get no clues as to what
went wrong. This makes it very hard to debug. Part of the reason
for this is that we were worried about it entering a recursive
backtracing loop, however not having any messages at all when
something goes awry is a bit of a problem.
To address this, have the backtracing code set an environment
variable to indicate that we're backtracing, then if we crash with
that variable set, disable the backtracer for the subsequent run
of the backtracer. This means that if the initial backtracer
instance crashes, we will try to run at most one more instance to
backtrace *that*.
rdar://170801141
9c3c25f07f