Index: cmake/Modules/HandleOutOfTreeLLVM.cmake
===================================================================
--- cmake/Modules/HandleOutOfTreeLLVM.cmake
+++ cmake/Modules/HandleOutOfTreeLLVM.cmake
@@ -93,6 +93,8 @@
     endif()
   endif()
   if (LLVM_FOUND)
+    # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions.
+    set(LLVM_ENABLE_WARNINGS ON)
     include(AddLLVM OPTIONAL)
     include(HandleLLVMOptions OPTIONAL)
   endif()