[clang] [clang][Parser] "Better" error messages for invalid template template (PR #95726)

[via cfe-commits]

Sirraide wrote:

> So I dont end up making 100 commits rewording this: 

It’s fine if you do. We do squash+merge anyway, so the entire pr will end up as one commit either way ;Þ

> How about "Clang now diagnoses template template paramter missing `<` as typo for `typename`"

Maybe something like ‘Clang now suggests a fix-it to correct `template` in a template-template parameter with a missing `<` to `typename`’ seeing as we were already diagnosing this; the only thing that’s changed is that we now also provide a fix-it.


https://github.com/llvm/llvm-project/pull/95726