Rollup merge of #128402 - oli-obk:checked_attrs, r=compiler-errors

Attribute checking simplifications

remove an unused boolean and then merge two big matches into one

I was reviewing some attributes and realized we don't really check this list against the list of builtin attributes, so we "may" totally be missing some attributes that we should be checking (like the `coroutine` attribute, which you can just apply to random stuff)

```rust
#![feature(coroutines)]
#[coroutine]
struct Foo;
```

just compiles for example. Unless we check that the fallthrough match arm is never reached for builtin attributes, we're just going to keep forgetting to add them here, too. I can do that without the changes in this PR, but it seemed like a nice cleanup

Author: matthiaskrgr