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]
  };

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193049 91177308-0d34-0410-b5e6-96231b3b80d8