RawCommentList::addComment: fix the assertion so it actually checks that new...

RawCommentList::addComment: fix the assertion so it actually checks that new comment is after the last one (change Comments[0] to Comments.back()), and handle the case of two consecutive comments, e.g. /** *//* */.  There is already a testcase for that (but it didn't trigger the assert because the assert itself was wrong).

llvm-svn: 158882