clang-tidy: [use-override] Tweak messages.

It doesn't make sense to suggest 'virtual' as clang-tidy would complain
about that on the next iteration (we are never issuing warnings for the
base function).

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@214063 91177308-0d34-0410-b5e6-96231b3b80d8