Rust: Add placeholder declarations for &mut and *mut

[hvitved]

Adding these declarations now mean we will already have DBs with those entities available once we implement support for distinguishing mut/const types.

[Copilot]

Pull request overview

This PR adds placeholder struct declarations for RefMut and PtrMut to represent mutable reference (&mut) and mutable pointer (*mut) types in the Rust builtin types system. These additions prepare the codebase for future support of distinguishing between mutable and const types.

• Removed TODO comments for mut variants
• Added RefMut<TRefMut> and PtrMut<TPtrMut> struct declarations
• Updated test expectations to include the new types

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rust/tools/builtins/types.rs	Added RefMut and PtrMut placeholder structs for mutable reference and pointer types, removing previous TODO comments
rust/ql/test/library-tests/elements/builtintypes/BuiltinTypes.expected	Updated generated test expectations to include the new PtrMut and RefMut types in alphabetical order

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[paldepind]

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