Fix Subzero's LLVM bitcode reader to use a diagnostic handler.

This encorporate the changes introduced by llvm CL
https://codereview.chromium.org/1310883003 (see for details).

BUG=None
R=dschuff@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1312473006 .
1 file changed