llvm_tools: add a tool to bisect clang crashes

Add a tool that downloads clang crash diagnoses from
chrome-toolchain-artifacts/clang-crash-dignoses and send them to 4c
server for bisection.

BUG=chromium:1056904
TEST=local tests

Change-Id: I75081259705c236a20173fc591d080c6b0504cc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2325008
Tested-by: Jian Cai <jiancai@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
3 files changed