Auto merge of #127313 - cjgillot:single-expect, r=jieyouxu

Rewrite lint_expectations in a single pass.

This PR aims at reducing the perf regression from rust-lang/rust#120924 (comment) with drive-by simplifications.

Basically, instead of using the lint level builder, which is slow, this PR splits `lint_expectations` logic in 2:
- listing the `LintExpectations` is done in `shallow_lint_levels_on`, on a per-owner basis;
- building the unstable->stable expectation id map is done by iterating on attributes.

r? ghost for perf

Author: bors