Add -fsanitize=<sanitizers> argument to driver and frontend, and add

-fno-sanitize=<sanitizers> argument to driver. These allow ASan, TSan, and the
various UBSan checks to be enabled and disabled separately. Right now, the
different modes can't be combined, but the intention is that combining UBSan
and the other sanitizers will be permitted in the near future.

Currently, the UBSan checks will all be enabled if any of them is; that will be
fixed by the next patch.

llvm-svn: 167411