swift234

[DocComment] Add test for bogus image URLs and enable all tests

- Reenable doc comment extension tests by moving the inputs into the
  Inputs subdirectory. rdar://problem/21295424

- cmark could crash when getting an empty URL in an image element,
  because the URL was stored as a raw char * pointer. This was fixed in
  jgm/cmark pull request https://github.com/jgm/cmark/pull/38, so GitHub/master
  is pulled into our cmark clone.
  rdar://problem/21910368

Swift SVN r30475