rustdoc: Show use-site paths for unevaluated const array lengths
After rust-lang/rust#158171, I noticed a few additional issues with the rendering of consts in rustdoc. This PR fixes them. When rendering array lengths, avoid inlining the full definition of unevaluated constants via `print_inlined_const`. Instead, fall back to `Const::to_string()` for use-site rendering, matching the behavior already used for projections without bodies.
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