Restrict EII declarations to functions at lowering time

[enthropy7]

We tighten EII declaration resolution so that lower_path_simple_eii only accepts function‑like items (Fn, AssocFn, Ctor(_, Fn)) as valid EII targets. If name resolution points at something else (like a const with the same name), we now emit a direct error (“externally implementable items must refer to a function”) at the declaration site, which prevents bad DefIds from ever reaching compare_eii_function_types and turning into an ICE

this is more robust and root-cause oriented fix to #152337 issue and an alternate of my more simple PR #152365

test included

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[enthropy7]

previously I tightened lower_path_simple_eii for all EII paths, which also affected macro targets (eii_macro_path) and broke a number of existing EII tests. this revision restores lower_path_simple_eii to its original behavior and moves the function‑kind check into a dedicated lower_eii_decl_target used only for EII declarations, so we still reject non‑function declarations without changing the behavior of macro-based EII implementations and existing tests.

[petrochenkov]

This looks marginally better than #152365 to me.
Although the right place for reporting errors like "expected definition X, found definition Y" is rustc_resolve, see e.g. PathSource::is_expected.
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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[jdonszelmann]

r? me I'll look at this tomorrow morning but don't merge anything similar to this pr until I did

[jdonszelmann]

I agree with petrochenkov, this has to be solved using PathSource @enthropy7

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[enthropy7]

Moved EII declaration target validation from ast_lowering to resolve via PathSource (ExternItemImpl), so diagnostics now come from resolver

also updated tests

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[enthropy7]

@rustbot ready

rebased onto latest main. updated ExternItemImpl::is_expected to also accept Ctor(..) to preserve clean E0428 diagnostics for constructor name collisions.