Fix nonexistent BackedEnum in stubs

compatibility/BackedEnum.stub

@@ -0,0 +1,6 @@
+<?php
+
+interface BackedEnum
+{
+
+}

phpstan.neon

@@ -64,3 +64,9 @@ parameters:
 				- '#^Call to function method_exists\(\) with Doctrine\\DBAL\\Connection and ''createSchemaManager'' will always evaluate to true\.$#'
 				- '#^Call to an undefined method Doctrine\\DBAL\\Connection\:\:getSchemaManager\(\)\.$#'
 			path: tests/Platform/QueryResultTypeWalkerFetchTypeMatrixTest.php
+
+services:
+	-
+		class: PHPStan\BackedEnumStubExtension
+		tags:
+			- phpstan.stubFilesExtension

tests/BackedEnumStubExtension.php

@@ -0,0 +1,22 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan;
+
+use PHPStan\PhpDoc\StubFilesExtension;
+use const PHP_VERSION_ID;
+
+class BackedEnumStubExtension implements StubFilesExtension
+{
+
+	public function getFiles(): array
+	{
+		if (PHP_VERSION_ID >= 80100) {
+			return [];
+		}
+
+		return [
+			__DIR__ . '/../compatibility/BackedEnum.stub',
+		];
+	}
+
+}