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

[Balazs Benics via cfe-commits]

https://github.com/steakhal commented:

This is my first round of review. So far it looks good.
I didn't have much time, but that's all I had.

I think it would make sense to have tests for variants containing variants and playing with the active members. That would be interesting to see.

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