CI: have Clang-Tidy check our code

[agarny]

Unless I missed it, we don't have any test with Clang-Tidy enabled. And, as it happens, I wanted to check something and enabled Clang-Tidy (-DCLANG_TIDY=ON) in my local build and I got tons of issues. So, we might want to add a test where Clang-Tidy checks our code.