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

[Gábor Spaits via cfe-commits]

spaits wrote:

@steakhal we have also run the checker on multiple open source projects (a few weeks ago) and it did not crash. (It did not have any findings. There were only two C++ 17 projects and they had retrieved value from std::variant at most 20 times).

Could you please take another look at the commit and give feedback, when you'll have time for it?

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