Add diagtool-based test where we enforce that no additional warnings can be added to Clang without a -W flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137369 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 797c620..b88e49d 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -110,7 +110,7 @@
 
   add_dependencies(clang-test clang-test.deps)
   add_dependencies(clang-test.deps
-                 clang clang-headers c-index-test arcmt-test c-arcmt-test
+                 clang clang-headers c-index-test diagtool arcmt-test c-arcmt-test
                  )
 
 endif()