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

[Gábor Spaits via llvm-commits]

spaits wrote:

@steakhal thank you for the review. I have made the changes you have asked for.
 - Changed the name of header guard macro to reflect the path of the file
 - Used the new namespace opening syntax
 - Changed a function's argument, so it takes reference instead of pointer
   - It may would be a good task for me to see why the `checkRegionChanges` callback uses pointer instead of reference.
 - Moved a few functions into the class `CallEvent`
   - There are other functions I did not move into `CallEvent` because they are called on a `CallEvent`, cast it to a subclass then tell things about it. It would not be nice to have a function like this in the `CallEvent` super class. But if you think I should move them there then I can do that.

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