Clang tidy showcase

[teksturi]

This PR is just showcase what kind of things clang-tidy could do. So I am not suggesting we take all of these checks in use. Currently not buildable as it makes mistakes with some cases. It did also found many bugs. I have to send fixes to those first before doing real clang-tidy things.

We just need to get these thing automated so they do not happened again. Note that these are only checks which clang-tidy can fix automatically. There is also bunch of good checks which can give just warning.

This was done for #4348

[matejk]

@teksturi, this is excellent!

How would clang-tidy be automated?

Shall it be added as the last build target in CMake and make the build fail?

Or is it safe for automatic fixes to be applied also?

[obiltschnig]

We should enable clang-tidy checks as part of normal CI.
In the first step, just print the warnings but not fail. After all things are fixed, make the build step fail if clang-tidy reports issues. Probably need to clarify what warnings or errors would cause the build to fail also (maybe some are acceptable/unavoidable).
I don't want automatic fixes for now.