clang-format: Properly align ObjC string literals.

Before:
  NSString s = @"a"
               "b"
               "c";
  NSString s = @"a"
               @"b"
                @"c";

After:
  NSString s = @"a"
                "b"
                "c";
  NSString s = @"a"
               @"b"
               @"c";

This fixes llvm.org/PR23536.

llvm-svn: 237538