Restrict EII declarations to functions at lowering time
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 rust-lang/rust#152337 issue and an alternate of my more simple PR rust-lang/rust#152365
test included
Jonathan Brouwer
5b16a31a0e