clang-format: Understand ObjC boxed expressions.

Before:
  [dictionary setObject:@(1)forKey:@"number"];
After:
  [dictionary setObject:@(1) forKey:@"number"];

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198920 91177308-0d34-0410-b5e6-96231b3b80d8