Engine: F*: interface mode: dropped_body on associated constants of inherent impls

[W95Psp]

Consider a struct X and an inherent impl block for X, as follows:

struct X;
impl X {
    const A: u8 = 0;
}

A is an associated constant for X.

When extracted with interface flags on to F*, this results in a let binding with a dropped_body () body.

Why this is not normal:
The current policy is that constant items that are marked "extract type-only" are actually fully translated: extracting a constant almost never yields issues, and is almost always useful in proof settings.

Plausible explanation for this bug:
Here, this policy doesn't apply probably because associated constants in inherent impls are translated with c_expr instead of c_body in thir_export.ml.