Properly handle new tombstone values that now appear as a result of the llvm change described at https://reviews.llvm.org/D81784.

Change-Id: I79dc5a72b651aa057104cd42b4773391df68125b
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2317730
Reviewed-by: Mark Mentovai <mark@chromium.org>
Reviewed-by: Nelson Billing <nbilling@google.com>