Fix ICE #152205: upvar analysis with associated type projections

[rynewang]

Summary

Fix ICE in closure capture migration analysis when encountering associated type projections.

Problem

#![deny(rust_2021_incompatible_closure_captures)]
struct Foo;
struct S;
impl Drop for S { fn drop(&mut self) {} }
struct U(<Foo as NewTrait>::Assoc);
fn test(u: U) { let _ = || { let _x = u.0.0; }; }
trait NewTrait { type Assoc; }
impl NewTrait for Foo { type Assoc = (S,); }

ICEs at upvar.rs:1685 with unreachable!().

Fix

Replace three unreachable!() calls with conservative fallbacks:

• unreachable!() in filter_map → None
• _ => unreachable!() → _ => false

Safe because this only affects the edition migration lint — conservative false means we might miss a warning, which is better than an ICE.

Includes regression test.

Fixes rust-lang#152205