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
Now that Darwin is provided by the system, we no longer need to disable the
system module maps in favour of our own, which also resolves a few other
problems that doing that was creating.
rdar://137201928
Using `SwiftShims` is undesirable - it creates all kinds of build issues,
and means shipping the `_SwiftBacktracing.h` header in the SDK, which is
not necessary.
While we're doing this, add the necessary definitions for reading ELF
and DWARF information.
rdar://110261712
