clang-format: Add @s when breaking NSString literals.

While it is allowed to not have an @ on subsequent lines, it seems
general practice to add them. If undesired, the code author can easily
remove them again and clang-format won't re-add them.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198871 91177308-0d34-0410-b5e6-96231b3b80d8