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

[Harald van Dijk via cfe-commits]

hvdijk wrote:

At some point this seems to have been fixed, so I have updated this PR to just add the test and an assertion, nothing else, to make sure we do not regress.

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