We need a minimum target of 15.7 or below because of CI, so add a new
`BacktracingDT 6.2` define. This is a bit of a hack for now to
unblock CI upgrades; we can go and clean things up and perhaps
generalise this mechanism later.
rdar://172129091
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.