[MXNET-859] Add a clang-tidy stage to CI

[KellenSunderland]

Description

This integrates clang-tidy into our CI system. Clang-tidy is a modern c++ linting and static-analysis tool that can find issues in our native codebase. It can also be used to modernize the codebase, suggesting improvements to adopt features that have been added to the C++11 standard.

This initial integration marks many rules as warnings and only targets cpu builds for our core native codebase. I'd like to try and introduce this as a minimum useful change. As we fix warnings we can move rules to errors in the configuration to protect against regressions.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

[KellenSunderland]

Todo: switch from args to yaml config file.

[marcoabreu]

Let me know when you are ready

[KellenSunderland]

@marcoabreu: Ready to merge.