Don't emit -Wnon-virtual-dtor on final classes, since it's not a problem there.

The base class is the culprit/risk here - a sealed/final derived class
with virtual functions and a non-virtual dtor can't accidentally be
polymorphically destroyed (if the base class's dtor is protected - which
also suppresses this warning).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208449 91177308-0d34-0410-b5e6-96231b3b80d8