[asan] Use asan exitcode=0 option to always succeed a test run.

Summary:
halt_on_error-torture.cc intermittently fails on ppc64be, let's try to
collect more info on failures.

Reviewers: eugenis

Subscribers: kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D33912

llvm-svn: 304731