[ubsan] Stop cast-overflow.cpp test leaking undefined behaviour into the exit code.

Summary:
ubsan was correctly catching the undefined behaviour but lit's shell was
failing the test anyway because the exit code was non-zero as a result of the
undefined behaviour.

This fixes the test on a mips-linux-gnu target.

Reviewers: samsonov

Reviewed By: samsonov

Subscribers: samsonov, llvm-commits, rsmith, sagar

Differential Revision: http://reviews.llvm.org/D9155

llvm-svn: 235518