Rollup merge of #111039 - compiler-errors:foreign-span-rpitit, r=tmiasko

Encode def span for foreign return-position `impl Trait` in trait

Fixes #111031, yet another def-span encoding issue :/

Includes a smaller repro than the issue, but I can confirm it ICEs:

```
query stack during panic:
#0 [def_span] looking up span for `rpitit::Foo::bar::{opaque#0}`
#1 [object_safety_violations] determining object safety of trait `rpitit::Foo`
#2 [check_is_object_safe] checking if trait `rpitit::Foo` is object safe
#3 [typeck] type-checking `main`
#4 [used_trait_imports] finding used_trait_imports `main`
#5 [analysis] running analysis passes on this crate
```

Luckily since this only affects nightly, this desn't need to be backported.

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -837,11 +837,12 @@ fn should_encode_span(def_kind: DefKind) -> bool {
         | DefKind::AnonConst
         | DefKind::InlineConst
         | DefKind::OpaqueTy
+        | DefKind::ImplTraitPlaceholder
         | DefKind::Field
         | DefKind::Impl { .. }
         | DefKind::Closure
         | DefKind::Generator => true,
-        DefKind::ForeignMod | DefKind::ImplTraitPlaceholder | DefKind::GlobalAsm => false,
+        DefKind::ForeignMod | DefKind::GlobalAsm => false,
     }
 }