Enable LLVM_ENABLE_WARNINGS when building standalone out of tree

When built within the llvm runtimes directory, the runtimes
CMakeLists.txt adds the same.

Differential Revision: https://reviews.llvm.org/D56979

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@351873 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/Modules/HandleOutOfTreeLLVM.cmake b/cmake/Modules/HandleOutOfTreeLLVM.cmake
index e50d026..3b19c5c 100644
--- a/cmake/Modules/HandleOutOfTreeLLVM.cmake
+++ b/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()