[clang] [llvm] [analyzer] New checker: optin.core.UnconditionalVAArg (PR #175602)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

After merging https://github.com/llvm/llvm-project/pull/182058 (Add BranchCondition callback to 'switch') I hope that I can finally merge this PR as well.

I merged a recent `main` into the branch of this PR and resolved some mild conflicts in documentation. I'm fairly confident that this is sufficient for correct behavior and I don't need to add any additional patches here; but for the sake of safety I'm redoing the analysis of open source projects (which uncovered the problems with `switch` statements).

> I found an example of this during the analysis of the `sqlite` project, which confirms that consistently testing patches on a set of real-world projects is a good thing to do.
> 
> I'll come up with a fix for this issue within a few days.

This "few days" turned into a month, but it was an interesting journey :sweat_smile: and I learned a lot about the analyzer along the path.

https://github.com/llvm/llvm-project/pull/175602