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

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DonatNagyE 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.

The most urgent ones are https://github.com/llvm/llvm-project/pull/66207 (a "real" review) and https://github.com/llvm/llvm-project/pull/66647 (where nothing significant changed since you reviewed it last time). I hope that you can do those and this one.

Moreover, I'm planning to merge the commit https://github.com/llvm/llvm-project/pull/67572 because it's just an NFC cleanup and @gamesh411 already reviewed it.

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