clang-tidy: [misc-use-override] Correctly handle defaulted destructors.

Also, minor implementation and test fixes.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@211345 91177308-0d34-0410-b5e6-96231b3b80d8
Merge pull request #12 from llvm-mirror/master

clang-tidy: [misc-use-override] Correctly handle defaulted destructors.