clang-tidy use override: Don't generate incorrect warnings without fixes

Add basic testing for the emitted diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@210171 91177308-0d34-0410-b5e6-96231b3b80d8
clang-tidy: Make use-override tests stricter.

As per post review comment.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@210173 91177308-0d34-0410-b5e6-96231b3b80d8