Add FileLocator::locate stub and test

src/Stubs/common/FileLocator.stubphp

@@ -0,0 +1,13 @@
+<?php
+
+namespace Symfony\Component\Config;
+
+class FileLocator implements FileLocatorInterface
+{
+    /**
+     * @psalm-template TFirst as bool
+     * @psalm-param TFirst $first
+     * @psalm-return (TFirst is true ? string : array)
+     */
+    public function locate(string $name, string $currentPath = null, bool $first = true) {}
+}

tests/acceptance/acceptance/FileLocator.feature

@@ -0,0 +1,33 @@
+@symfony-common
+Feature: FileLocator locate
+
+  Background:
+    Given I have Symfony plugin enabled
+
+  Scenario: FileLocator locate method return type should be `string` when third argument is true
+    Given I have the following code
+      """
+      <?php
+      function test(): string
+      {
+        $locator = new \Symfony\Component\Config\FileLocator(__DIR__);
+
+        return $locator->locate(__FILE__);
+      }
+      """
+    When I run Psalm
+    Then I see no errors
+
+  Scenario: FileLocator locate method return type should be `array` when third argument is false
+    Given I have the following code
+      """
+      <?php
+      function test(): array
+      {
+        $locator = new \Symfony\Component\Config\FileLocator(__DIR__);
+
+        return $locator->locate(__FILE__, null, false);
+      }
+      """
+    When I run Psalm
+    Then I see no errors