Comment parsing tests: move tests where they belong

Move C++-specific tests that were checking if we attach a base class comment to
a derived class to an existing test comment-to-html-xml-conversion.cpp.  Note
that the original testing approach was not actually testing the class--comment
relationship.  It only checked that we attached the comment *somewhere*.

The rest of subclass-comment.mm should be also moved elsewhere.

llvm-svn: 188658