Set '-target' flag in the test checking the MacOS include dir

To fix a buildbot failure on PS4, see
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/42251

The test was added in r351222 and aims to check only a particular
Mac configuration. However it relied on the default compiler target
by default, therefore unintentionally failing on PS4.

llvm-svn: 351334

clang/test/Tooling/clang-check-mac-libcxx-fixed-compilation-db.cpp

@@ -10,7 +10,7 @@
 //
 // RUN: cp $(which clang-check) %t/mock-libcxx/bin/
 // RUN: cp "%s" "%t/test.cpp"
-// RUN: %t/mock-libcxx/bin/clang-check -p "%t" "%t/test.cpp" -- -stdlib=libc++
+// RUN: %t/mock-libcxx/bin/clang-check -p "%t" "%t/test.cpp" -- -stdlib=libc++ -target x86_64-apple-darwin
 
 #include <mock_vector>
 vector v;