compiler_wrapper: redirect clang crash dignoses

Redirect all diagnoses of clang crashes to a dedicated directory using
fcrash-diagnostics-dir so we can upload these files to the cloud. The
flag was added to clangPostFlags to overwite redirection done on package
level.

BUG=chromium:1056904

TEST=None

Change-Id: I78bfc77d7f3af8556b4308f2e71407d627627747
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2251038
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Jian Cai <jiancai@google.com>
Tested-by: Jian Cai <jiancai@google.com>
40 files changed