[clang] [analyzer] Fix StdVariantChecker crash on std::get with a non-ptr arg (PR #210167)

[Balázs Benics via cfe-commits]

https://github.com/steakhal approved this pull request.

The fix looks good to me. I had a minor - unrelated comment while I was there.

https://github.com/llvm/llvm-project/pull/210167