On-crash backtracing is basically there for 64-bit Windows. It
won't work on 32-bit because of a Swift compiler issue, and there
is a little more work to do yet, but it is now working!
rdar://101623384
Changed the way swift-backtrace works for JSON, so that it captures
into a CrashLog, then writes to JSON from the CrashLog. Separating out
the two sets of logic so that writing the CrashLog to JSON becomes
simpler and more modular.