refactor: derive const type from ConstValueId

[lisenokdonbassenok]

Summary

This change makes the Const generator derive the variable type directly from ConstValueId::ty(ctx.db)
and removes the redundant ty field. All call sites of generators::Const in lowering are updated to
stop passing a type explicitly and rely on the type embedded in the const value.

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Type of change

• New feature

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Why is this change needed?

Previously the lowering Const generator duplicated type information: it stored both a ConstValueId and an explicit semantic::TypeId. This opened the door for potential divergence between the value's actual type and the manually passed type.

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[reviewable-StarkWare]

This change is 

[orizi]

@orizi made 1 comment.
Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @lisenokdonbassenok and @TomerStarkware).

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a discussion (no related file):
not sure it is actually better, as it adds an additional call, where in all cases the type was already available.
@TomerStarkware ?

[orizi]

Closing.