[test] FileCheck-ify a test to avoid a spurious failure, NFC
[test] FileCheck-ify a test to avoid a spurious failure, NFC

The path to one of my source trees contains 'builtin' as a substring, so
this test failed. Fix it with FileCheck.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@309460 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang