Experiment computing lint_levels by definition

[cjgillot]

The lint_levels query traverses the whole crate to assign a set of lint and allowances to each HirId.

However, most incremental modifications to a crate don't involve many items. Computing lint_levels independently for each definition will help reduce the effect of recomputing it. The objective is to avoid lowering of some definitions once #88186 is merged.

Steps: not defined yet. Please make contact on Zulip to discuss and define them.

[fbrunodr]

I have not contributed yet to rust, but I am willing to help if you want!

[fee1-dead]

@rustbot claim