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

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Veeloxfire wrote:

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

Aha I didnt realise!

> seeing as we were already diagnosing this; the only thing that’s changed is that we now also provide a fix-it.

I agree but also now it fully parses as if `typename` so that means code which previously did `template<template A> void foo(A)` will now parse `A` as a type (instead of skipping) and not error the second time for `A` not existing

This change isnt reflected in that, but I think its probably good enough and not worth worrying about

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