Add the ability to specify a filename or directory name as the output-to
setting in `SWIFT_BACKTRACE`. If the option is set to a directory name,
generate a unique filename in that directory using the process name,
process ID and timestamp.
rdar://136977833
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
We have some problems on Linux where Glibc pulls in `<elf.h>` and then
we end up with conflicting definitions. Fix by using C++ interop and
putting our definitions into a namespace.
rdar://137201928
While I was doing this, it turns out Saleem was fixing things to avoid
having to patch the Windows include directories, which is awesome but
necessitates an extra change to the backtracing stuff to make the build
not fail on Windows.
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
