A pretty big overhaul of the TryFallbackUnwindPlan method in
RegisterContextLLDB.  I have core files of half a dozen tricky
unwind situations on x86/arm and they're all working pretty much
correctly at this point, but we'll need to keep an eye out for
unwinder regressions for a little while; it's tricky to get these
heuristics completely correct in all unwind situations.

<rdar://problem/18937193>