[DataFormatters] Implement summary for __NSDictionary0.

Before the patch:

(lldb) frame var emptyDictionary
(__NSDictionary0 *) emptyDictionary = 0x0000000100304420

After:

(lldb) frame var emptyDictionary
(__NSDictionary0 *) emptyDictionary = 0x0000000100304420 0 key/value pairs

There's nothing much else we can do, as this is always empty by
definition.

<rdar://problem/34806516>

llvm-svn: 327587