[debugger] Format MaybeDangling in async-backtrace

This appears around some Future objects in newer toolchains after around
https://fxrev.dev/1476058. There are no expected changes to output
before the new toolchain.

Fixed: 476220782
Change-Id: I288d785a468b0ed70edfdb1ecda9f5d86a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1476816
Reviewed-by: Nathaniel Manista <nathaniel@google.com>
Fuchsia-Auto-Submit: Jacob Rutherford <jruthe@google.com>
Commit-Queue: Jacob Rutherford <jruthe@google.com>