Update LLVM_PREBUILT_VERSION to clang-r344140
am: 775d7fb224

Change-Id: Ic97695eed605965269a75858bc837d95ca8c61ef
tree: 8e33f4b5810c78074a110c0c4b59c0efd451e057
  1. darwin-x86/
  2. linux-x86/
  3. build-prebuilts.sh
  4. manifest.xml