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
Don't delete the OS declaration of `exit` because the concurrency shims aren't always imported, and so the shim declaration might not always be available.
Don't override the OS declaration of `exit` in the concurrency shims since we can't just delete the OS one. Instead, set up internal shims just for building Concurrency that forward declares `exit`.
The test were enabled even if Backtracing was not enabled in the build
settings. Backtracing is enabled with the build script, but not in the
default CMake configuration.
