Various build fixes for clang and rust builds

[mhlakhani]

This PR fixes the clang build (for the thrift compiler) and makes the Rust tutorial build pass as well.

The C++ changes fix inconsistent override compiler warnings, and the Rust change fixes a clippy warning about an unnecessary dereference.

I tested this with the below, which now passes (previously failed):

./configure --disable-dependency-tracking --without-python --without-ruby --without-perl  --without-py3 --without-swift --without-nodejs
make -j16

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[emmenlau]

I have not compiled this myself, but if your compiler happily accepts the C++ changes, they should be good. Adding override can only help, or break the build, so if the build still works, it should only have helped :-)

So: C++ side looks good to me. Review of the Rust change, anyone?