Hitting `D` when in the backtracer should do the same on Windows
that it does on macOS, namely launching `lldb` and attaching it to
the crashed program.
rdar://101623384
On-crash backtracing is basically there for 64-bit Windows. It
won't work on 32-bit because of a Swift compiler issue, and there
is a little more work to do yet, but it is now working!
rdar://101623384