[clang] [clang-tools-extra] [Clang][Sema] Diagnose current instantiation used as an incomplete base class (PR #92597)

[Matheus Izvekov via cfe-commits]

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

The change itself looks pretty good, but note that GCC only warns about this, and more importantly MSVC still accepts the code without complaint.

So this means that we could find out from user feedback that we will need to keep the old behavior around behind a flag and for ms-compat mode.

But I would be much in favor of going ahead, only making that change later depending on impact.

https://github.com/llvm/llvm-project/pull/92597