See tools/clang/scripts/update.sh. This'll be used to run ASan on the bots.

BUG=469928

Change-Id: I6b5093c2db21ad4ed742852944e77a6b32e29e29
Reviewed-on: https://boringssl-review.googlesource.com/4402
Reviewed-by: Adam Langley <agl@google.com>