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

[eddyb]

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:

// This should always warn:
use std::u32;
// If only the builtin type is used:
fn main() { let _: u32; }

cc @petrochenkov

[ollie27]

Duplicate of #40774

[Mark-Simulacrum]

Closing as a duplicate.