[ASan] A speculative attempt to fix a flaky test on ppc64be.

Summary:
As mentioned in test/msan/fork.cc, if test output is redirected to a file
(as opposed to being piped directly to FileCheck), we may lose some "done"s due to
a kernel bug: https://lkml.org/lkml/2014/2/17/324, so let's pipe the
output of the test.

Reviewers: eugenis

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D33915

llvm-svn: 304744