Add a REQUIRES: darwin line for a mac test.

This test, apparently for macs, fails on Windows as lit can't emulate
the shell subprocess $(which...) correctly. Some other netbsd and linux
buildbots also fail here. Limit to macs as a temporary workaround.

llvm-svn: 351360

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

@@ -11,6 +11,6 @@
 // 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++ -target x86_64-apple-darwin
-
+// REQUIRES: system-darwin
 #include <mock_vector>
 vector v;