Mentions of builtin types mark an import with the same name as used.

Found when implementing #44240: turns out `panic!()` in a module marks `std::u32` imports as used, even if they aren't actually needed.
Simple testcase:
```rust
// This should always warn:
use std::u32;
// If only the builtin type is used:
fn main() { let _: u32; }
```
cc @petrochenkov