Add VTablePlaceholderDecl (but don't hook it up to anything yet).

lib/IDE/CodeCompletion.cpp

@@ -535,6 +535,7 @@ CodeCompletionResult::getCodeCompletionDeclKind(const Decl *D) {
   case DeclKind::EnumCase:
   case DeclKind::TopLevelCode:
   case DeclKind::IfConfig:
+  case DeclKind::VTablePlaceholder:
     llvm_unreachable("not expecting such a declaration result");
   case DeclKind::Module:
     return CodeCompletionDeclKind::Module;