[llvm] [analyzer] Add std::variant checker (PR #66481)

[Balazs Benics via llvm-commits]

steakhal wrote:

@DonatNagyE I can see you all have a lot of PRs. I have limited time, but I think I can do two this week. Maybe one next week.
In what order should I go through the PRs?
I guess, this one should be in the lucky two.

https://github.com/llvm/llvm-project/pull/66481