Use explicit if and restore lost FileCheck error code checking.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@212546 91177308-0d34-0410-b5e6-96231b3b80d8
As funny as it might look, this wasn't what I intended to test.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@212549 91177308-0d34-0410-b5e6-96231b3b80d8