Use ASSERT_THAT_ERROR instead of logAllUnhandledErrors/exit

Summary: ASSERT_THAT_ERROR looks like the intended helper for use in tests.

Reviewers: plotfi, jkorous, compnerd

Subscribers: mgorny, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65853

llvm-svn: 368399