Fix unused_variables typo suggestions for const patterns

[onehr]

Closes #147595

This fixes unused_variables typo suggestions that could print replacement
patterns which are not valid from the binding site.

The fix keeps the existing def_path_str output when it is already valid, but
adjusts two local-item cases:

• items outside the binding module are printed with crate::...;
• items nested inside the current body are printed relative to that body.

It also suppresses const typo suggestions for bare let x = ... bindings,
because replacing the binding with a const path creates a refutable pattern.
Unit ADT typo suggestions are left unchanged.

The regression test covers 2015 and 2021 editions, cross-module consts,
same-module consts, function-local consts, cross-module variants, and bare
let const initializers.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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