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

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).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158882 91177308-0d34-0410-b5e6-96231b3b80d8