Rust: Handle variables introduced in if-let guards

[hvitved]

DCA is great: Improved performance on rust and a lot of rust/unused-variable false positives removed.

[Copilot]

Pull Request Overview

This PR improves the handling of variable scoping in match arm guards with nested let patterns in Rust. The key change refactors how variable scopes are managed for match arms to properly handle cases where variables declared in the match arm pattern can be shadowed within the guard expression itself.

Key changes:

• Refactored MatchArmScope from an abstract class with two implementations into two separate scope classes
• Introduced ConditionScope for match arm guards to properly handle variables declared in nested let expressions within guards
• Updated the variable declaration logic to correctly determine variable scope and ordering for match arm patterns

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
rust/ql/test/library-tests/variables/main.rs	Added test case match_pattern16() demonstrating variable shadowing in match arm guards with nested let patterns
rust/ql/test/library-tests/variables/variables.expected	Updated expected test results to include the new test case variables and their accesses
rust/ql/test/library-tests/variables/Ssa.expected	Updated SSA analysis expected results for the new test case
rust/ql/lib/codeql/rust/elements/internal/VariableImpl.qll	Refactored scope classes and variable declaration logic to properly handle match arm guard scoping

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[geoffw0]

Code changes LGTM.

The improvement to rust/unused-variable results is very welcome.