Reflection: Decode imported Objective-C classes and protocols as their own TypeRef

Right now we expect that every class and protocol has a field
descriptor that tells us if the entity is @objc or not.

For imported types, the descriptor will not exist if we did not
directly emit a field whose concrete type contains the imported
type. For example, in lldb, we might have a generic type whose
runtime substituted type includes an imported type.

In this case, TypeLowering would fail to produce a layout because
it did not find a field descriptor for the imported type.

A better approach is to have the TypeDecoder call a different
factory method for imported types, and handle them specially in
TypeLowering, bypassing the field type metadata altogether.

lib/RemoteAST/RemoteAST.cpp

@@ -438,6 +438,16 @@ public:
     return createNominalType(typeDecl, /*parent*/ Type());
   }
 
+  ProtocolDecl *createObjCProtocolDecl(StringRef name) {
+    auto typeDecl =
+        findForeignNominalTypeDecl(name, /*relatedEntityKind*/{},
+                                   ForeignModuleKind::Imported,
+                                   Demangle::Node::Kind::Protocol);
+    if (auto *protocolDecl = dyn_cast_or_null<ProtocolDecl>(typeDecl))
+      return protocolDecl;
+    return nullptr;
+  }
+
   Type createForeignClassType(StringRef mangledName) {
     auto typeDecl = createNominalTypeDecl(mangledName);
     if (!typeDecl) return Type();