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

[Gábor Spaits via cfe-commits]

spaits wrote:

@steakhal I talked with @DonatNagyE and @dkrupp . We came to the conclusion, that the checker is only an alpha checker, would also not mean breaking changes to the analyzer and would not bring false positives. It also has been reviewed by @DonatNagyE . I would like to merge it in this form and later iterate on it. Would it be okay for you? 

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