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
Reorganise the thread locals for the backtracing code in the `Runtime` module so
that there’s only one set of them, with everything else hanging off that. This
also reduces code duplication and consumption of thread local variable space.
Fix the `DefaultSymbolLocator`’s shared instance to be thread local.
rdar://171432566