Fix this test case for CMake builds after r126502, which sneakily  changed the actual executable name to clang-<version>.
Fix this test case for CMake builds after r126502, which sneakily  changed the actual executable name to clang-<version>.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@126560 91177308-0d34-0410-b5e6-96231b3b80d8

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