We should have a tool that finds unused diagnostic IDs

While working on #67075, I found several stale diagnostics that are declared in Diagnostics*.def files, but are never used anywhere in the compiler. I deleted most of the ones I found, but I’m sure there are many more.

We should have a tool that scans lib/ and include/ for uses of diagnostic IDs and flags any that are never actually used. This could perhaps be a test that would fail if there are unused diagnostics.

This might be a good first issue for a new contributor who has reasonably strong Python or Swift skills but doesn’t have any experience with compilers yet.