Don't look for non-type-level assoc consts when checking trait object types

[ViewWay]

Fixes #153731.

This is a cherry-pick of PR #153738.

Problem

PR #150843 introduced a regression where eager type aliases containing dyn Trait (with associated consts) would incorrectly report 'trait is not dyn compatible' errors. This affected 254 crates depending on old bitvec versions.

Root Cause

When checking for unspecified assoc items in trait object types, we were filtering for all associated consts (is_const()). But eager type aliases don't get wfchecked at definition site, which broke the assumption that dyn-incompatible traits would be rejected early.

Fix

Changed is_const() to is_type_const() to only check type-level associated consts, not regular associated consts. Regular associated consts cannot be bound in trait object types anyway, so checking them was unnecessary.

[rustbot]

HIR ty lowering was modified

cc @fmease

Some changes occurred in compiler/rustc_sanitizers

cc @rcvalle

[rustbot]

r? @chenyukang

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[rustbot]

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[fmease]

@ViewWay I have no idea what you're trying to accomplish here but it makes no sense whatsoever for you to to cherry-pick / copy the commit I authored in open PR RUST-153906. Please cease this behavior immediately!

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