Add matches_with_unrelated_if lint

[Souradip121]

Fixes #16719

What this PR does

Adds a new style lint matches_with_unrelated_if that warns when
matches!(expr, Pattern if guard) is used and the guard does not
reference any variable introduced by Pattern.

Motivation

The if guard inside matches! implies a relationship between the
pattern bindings and the condition. When the guard is independent of the
pattern, the grouping is misleading. Moving it out makes the intent clearer:

// Before (misleading — `f.is_bar()` has nothing to do with `Some(_)`)
let _ = matches!(f.foo(), Some(_) if f.is_bar());

// After (the two checks are clearly independent)
let _ = matches!(f.foo(), Some(_)) && f.is_bar();

This also helps catch cases where the user intended to use a pattern
binding in the guard but made an oversight (e.g. typed f.is_bar()
instead of x.is_bar() when the pattern was Some(x)).

Implementation

• New file: clippy_lints/src/matches/matches_with_unrelated_if.rs
• Registered in declared_lints.rs and matches/mod.rs
• Category: style, machine-applicable suggestion

Tests

Added tests/ui/matches_with_unrelated_if.rs covering:

• Guard uses an outer variable (should lint)
• Pattern has bindings that are never used in the guard (should lint)
• Pattern has no bindings, guard uses outer variable (should lint)
• Guard correctly uses the pattern binding (no lint)
• No guard at all (no lint)
• Guard uses both a binding and an outer variable (no lint)

changelog: new lint [matches_with_unrelated_if]

[rustbot]

r? @Jarcho

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The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
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