fix manual_ignore_case_cmp chars iterator cmp

[cyphercodes]

changelog: [manual_ignore_case_cmp]: detect case-folded .chars() iterators compared with Iterator::cmp

Fixes #16961

Summary

• Detect .chars().map(|c| c.to_ascii_lowercase()/to_ascii_uppercase()).cmp(...).is_eq() forms.
• Also handle the corresponding .is_ne() form by suggesting !eq_ignore_ascii_case.
• Add UI coverage for borrowed strings, owned strings, and the to_str().is_some_and(...) closure reproducer.

Tests

• cargo fmt --check -- clippy_lints/src/manual_ignore_case_cmp.rs tests/ui/manual_ignore_case_cmp.rs clippy_utils/src/sym.rs
• TESTNAME=manual_ignore_case_cmp cargo uibless
• TESTNAME=manual_ignore_case_cmp cargo uitest
• cargo test --test dogfood
• git diff --check

[rustbot]

r? @samueltardieu

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The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
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• Random selection from Jarcho, dswij, llogiq, samueltardieu