Some symbolication frameworks have a symbol cache; we probably don't want
to use that for test cases, to avoid running into problems where the cache
holds stale information.
rdar://105409147
Fixed the colours so that they work with all of the default Terminal
presets. Also changed things so that when colour is off, we only use
ASCII characters in our source code displays.
rdar://105452194
This is Swift's external backtracer. It's written in Swift, and it's
responsible for generating nice backtraces when Swift programs crash.
It makes use of the new `_Backtracing` library, which is also (mostly,
aside from some assembly language) implemented in Swift.
rdar://103442000
