rm ItemKind::OpaqueTy

This introduce an additional collection of opaques on HIR, as they can no
longer be listed using the free item list.

compiler/rustc_passes/src/check_attr.rs

@@ -814,7 +814,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             | Target::Mod
             | Target::GlobalAsm
             | Target::TyAlias
-            | Target::OpaqueTy
             | Target::Enum
             | Target::Variant
             | Target::Struct
@@ -1328,7 +1327,6 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         ) {
             let article = match target {
                 Target::ExternCrate
-                | Target::OpaqueTy
                 | Target::Enum
                 | Target::Impl
                 | Target::Expression