Enable `clang-tidy` in CI

[VivekPanyam]

#303 added support for running clang-tidy locally (./tools/tidy.sh), but we don't currently run it in CI as master does not pass all checks yet.

This issue tracks fixing all existing clang-tidy issues so we can enable it in CI.

[maifeeulasad]

Willing to work on this one. Can I get assigned to this?