OpaqueType: model verification and name import

Teach Clift's verify-against-model and ImportDescriptiveInfo
visitors about types whose handle has the OpaqueType rank: the
verifier rejects non-struct types with such handles, and the name
importer fills in the type's mutable name with
NameBuilder::opaqueTypeName based on the wrapped byte size.

lib/CliftImportModel/ImportDescriptiveInfo.cpp

@@ -217,6 +217,10 @@ public:
       return visitHelperStructType(mlir::cast<clift::StructAttr>(Attr), *L);
     }
 
+    if (auto L = pipeline::locationFromString(rr::OpaqueType, T.getHandle())) {
+      return visitOpaqueType(mlir::cast<clift::StructAttr>(Attr), *L);
+    }
+
     revng_abort("Unsupported type location");
   }
 
@@ -390,6 +394,13 @@ private:
     return mlir::success();
   }
 
+  mlir::LogicalResult
+  visitOpaqueType(clift::StructAttr ST,
+                  const pipeline::Location<decltype(rr::OpaqueType)> &L) {
+    ST.getMutableName().setValue(NameBuilder.opaqueTypeName(ST.getSize()));
+    return mlir::success();
+  }
+
   //===----------------------- Operation name import ----------------------===//
 
   auto getModelFunction(llvm::StringRef Handle) {

lib/CliftImportModel/Verify.cpp

@@ -233,6 +233,11 @@ private:
         return error() << "Clift ModuleOp contains a non-struct type with "
                           "an ArtificialStruct handle: '"
                        << Type.getHandle() << "'";
+    } else if (auto L = GetLocation(ranks::OpaqueType)) {
+      if (not mlir::isa<clift::StructType>(Type))
+        return error() << "Clift ModuleOp contains a non-struct type with "
+                          "an OpaqueType handle: '"
+                       << Type.getHandle() << "'";
     } else {
       return error() << "Clift ModuleOp contains a DefinedType with "
                         "an invalid handle: '"