Index: cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- cmake/modules/HandleLLVMOptions.cmake
+++ cmake/modules/HandleLLVMOptions.cmake
@@ -559,6 +559,8 @@
 
   # Enable -Wstring-conversion to catch misuse of string literals.
   add_flag_if_supported("-Wstring-conversion" STRING_CONVERSION_FLAG)
+else()
+  append("-w" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
 endif (LLVM_ENABLE_WARNINGS AND (LLVM_COMPILER_IS_GCC_COMPATIBLE OR CLANG_CL))
 
 macro(append_common_sanitizer_flags)