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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188658 91177308-0d34-0410-b5e6-96231b3b80d8