This doesn't have a working symbolicator yet, but it does build and
it can obtain a basic backtrace.
It also doesn't include a working `swift-backtrace` program yet.
rdar://101623384
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
Alastair Houghton