Avoid duplicate `large_assignments` lints By checking for overlapping spans. This PR does the "reduce noisiness" task in #83518. r? `@oli-obk` who added E-mentor and E-help-wanted and wrote the initial code. (The fix itself is in https://github.com/rust-lang/rust/pull/114774/commits/dc82736677a134a1b52def496db111681c053e82. The two commits before that are just small refactorings.)