Enable cmark smart punctuation in doc comments

The following unescaped sequences will be rendered with cmark's "smart
punctuation" rules, roughly:

- Closed pairs of single/double straight quotes -> curly quotes
- Double dash: en-dash
- Triple dash: em-dash
- Three periods -> ellipsis character

rdar://problem/24794606

Author: bitjammer

lib/Markup/Markup.cpp

@@ -320,7 +320,7 @@ parseElement(MarkupContext &MC, LineList &LL, ParseState State) {
 Document *llvm::markup::parseDocument(MarkupContext &MC, LineList &LL) {
   auto Comment = LL.str();
   auto CMarkDoc = cmark_parse_document(Comment.c_str(), Comment.size(),
-                                       CMARK_OPT_DEFAULT);
+                                       CMARK_OPT_SMART);
 
   if (CMarkDoc == nullptr)
     return nullptr;