Revert "Remove -Wweak-template-vtables"

Turns out there's still some code referencing this. No harm in keeping it in a
bit longer.

> as it was planned for removal in clang 15 and we're now past the branch point
>
> See https://github.com/llvm/llvm-project/issues/19107
>
> Differential revision: https://reviews.llvm.org/D118762

This reverts commit 564f9be1.