Rust: Refactor type inference to use new TypeItem class

[paldepind]

Just a quick go at getting some mileage out of the new TypeItem class in the type inference implementation.

[Copilot]

Pull request overview

This PR refactors the type inference implementation in Rust QL to use a new unified TypeItem class, consolidating the handling of structs, enums, and unions under a common abstraction.

• Introduces a new DataType class that replaces separate TStruct, TEnum, and TUnion type constructors with a single TDataType(TypeItem) constructor
• Replaces specific getter methods (getStruct(), getEnum(), getUnion()) with a unified getTypeItem() method across the codebase
• Refactors TupleLikeConstructor and Declaration classes to use the new abstraction, reducing code duplication

Reviewed changes

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

Show a summary per file

File	Description
rust/ql/lib/codeql/rust/internal/Type.qll	Introduces DataType class and makes StructType, EnumType, and UnionType subclasses; consolidates type constructors from TStruct, TEnum, TUnion to TDataType
rust/ql/lib/codeql/rust/internal/TypeInference.qll	Updates all type references to use TDataType constructor; refactors TupleLikeConstructor and Declaration classes to use getTypeItem() method and eliminate duplicate logic
rust/ql/lib/codeql/rust/internal/TypeMention.qll	Simplifies resolveRootType() by consolidating three separate type resolution cases into a single TDataType case
rust/ql/lib/codeql/rust/security/Barriers.qll	Updates barrier classes to use getTypeItem() instead of getStruct() or getEnum()
rust/ql/lib/codeql/rust/frameworks/rustcrypto/RustCrypto.qll	Updates algorithm name extraction to use getTypeItem() instead of getStruct()

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