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.
Once the API has gone through Swift Evolution, we will want to implicitly
import the _Backtracing module. Add code to do that, but set it to off
by default for now.
rdar://105394140
Adds a new swift_Backtracing library, with a corresponding _Backtracing
module, to the build. Also add some tests.
This is not public API at this point, but will be used by the external
backtracing program, `swift-backtrace`.
rdar://104336548
