Use the new module structure rather the old SwiftShims header. This
is much cleaner and lets us include operating system headers to get
the relevant definitions where possible.
Add code to support ELF and DWARF, including decompression using
zlib, zstd and liblzma if those turn out to be required and available.
rdar://110261712
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.
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
