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

[Harald van Dijk via cfe-commits]

hvdijk wrote:

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

Already updated the title, description, and commit message, but to be clear, it's not only adding a test, it's also adding the assertion. Are you still okay with that?

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