Replace \PDO::FETCH_* constants to indicate fetch mode with an enumeration

Closes #3487851

core/lib/Drupal/Core/Database/Statement/FetchAs.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\Core\Database\Statement;
+
+/**
+ * Enumeration of the fetch modes for result sets.
+ */
+enum FetchAs {
+
+  // Returns an anonymous object with property names that correspond to the
+  // column names returned in the result set. This is the default fetch mode
+  // for Drupal.
+  case Object;
+
+  // Returns a new instance of a requested class, mapping the columns of the
+  // result set to named properties in the class.
+  case ClassObject;
+
+  // Returns an array indexed by column name as returned in the result set.
+  case Associative;
+
+  // Returns an array indexed by column number as returned in the result set,
+  // starting at column 0.
+  case List;
+
+  // Returns a single column from the next row of a result set.
+  case Column;
+
+}