[clang] [clang-tools-extra] [RecursiveASTVisitor] Assert we skip implicit instantiations. (PR #110899)

[Erich Keane via cfe-commits]

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

Please update the commit message/title in github to better reflect that you're ONLY adding the tests, else this LGTM.

https://github.com/llvm/llvm-project/pull/110899