Rollup merge of #123998 - compiler-errors:opaque-ns, r=davidtwco

Opaque types have no namespace

Opaques are never referenced by name -- even when we have `type X = impl Sized;`, `X` is the name of the type alias, not the opaque.

compiler/rustc_hir/src/def.rs

@@ -207,7 +207,6 @@ impl DefKind {
             | DefKind::Enum
             | DefKind::Variant
             | DefKind::Trait
-            | DefKind::OpaqueTy
             | DefKind::TyAlias
             | DefKind::ForeignTy
             | DefKind::TraitAlias
@@ -234,7 +233,8 @@ impl DefKind {
             | DefKind::Use
             | DefKind::ForeignMod
             | DefKind::GlobalAsm
-            | DefKind::Impl { .. } => None,
+            | DefKind::Impl { .. }
+            | DefKind::OpaqueTy => None,
         }
     }