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
Adds a new swift_Backtracing library, with a corresponding _Backtracing
module, to the build. Also add some tests.
This is not public API at this point, but will be used by the external
backtracing program, `swift-backtrace`.
rdar://104336548
