The Swift backtracer's frame pointer unwinder cannot work on Linux
without this change, because the compiler omits the frame pointer from
the function in libSwift_Backtracing that actually captures the stack.

rdar://110260855

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

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

This is for compatibility, so that I can split up the PRs.
We'll remove it in the next PR.

rdar://110261712

There's a chance that pipes might perform a partial read; we should
handle that case.

rdar://110261712

The `status` argument to the `_swift_backtrace_demangle()` function
isn't especially useful, won't match behaviour on Windows, and we
actually don't use it in the Swift code that calls this SPI.

Remove it.

rdar://110261712

`__cxa_demangle()` is a rather unusual API; one of its "features" is that
the pointer you pass in must either be `nullptr`, in which case it
will call `malloc()` itself, _or_ it has to be a pointer to a block
of memory allocated with `malloc()`, because `__cxa_demangle()` may
`realloc()` it for you.

This seems to me to be something of a non-obvious footgun, so we never
pass the caller's pointer through to `__cxa_demangle()`, which lets them
decide how they want to allocate space.

rdar://110261712

…ule.

There's a separate declaration here because we can't include the `Backtrace.h`
header from inside `modules/Runtime/Runtime.h`.

rdar://110261712

The `_Backtracing` module has a number of private implementation only
imports that aren't used outside of the module and that don't require
any additional libraries (hence they aren't relevant to the outside
world).  `verify_all_overlays.py` needs to know about these when it
does its test, because it loadas the module as the main module, which
results in implementation only imports being required instead of
ignored.

rdar://110261712

We need a Linux specific `Target` implementation, and a couple of minor
tweaks to make things build everywhere.

rdar://110262673

This reverts commit 0d7c45d.

We don't need these here.  They were a leftover from previous code.

rdar://110262673