Move the backtracing code into a new Runtime module. This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.
rdar://124913332
Added some extra code to AddSwiftStdlib.cmake so executable targets can
specify target SDKs the same way libraries currently can.
Updated the Backtracing targets to specify just OS X for now.
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
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
