Updating a comment related to the implementation of -Woverloaded-virtual, and adding a FIXME to a test case. (Drive-by removal of trailing whitespace in the test case as well.)

No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214362 91177308-0d34-0410-b5e6-96231b3b80d8