clang-format: Improve formatting of ObjC dict literals.

Before:
  NSDictionary *d = @{ @"nam" : NSUserNam(), @"dte" : [NSDate date],
                       @"processInfo" : [NSProcessInfo processInfo]
  };

After:
  NSDictionary *d = @{
    @"nam" : NSUserNam(),
    @"dte" : [NSDate date],
    @"processInfo" : [NSProcessInfo processInfo]
  };

llvm-svn: 193049