[ticket/10751] Add sql_lower_text() to database abstraction layer.

On MSSQL, LOWER() can only be called on bounded strings (i.e. varchar or char).
So, in order to use it on a text column, we have to convert it to an
appropriate type. We do so using the SUBSTRING function.

PHPBB3-10751

phpBB/includes/db/mssqlnative.php

@@ -492,6 +492,14 @@ class dbal_mssqlnative extends dbal
 		return str_replace(array("'", "\0"), array("''", ''), $msg);
 	}
 
+	/**
+	* {@inheritDoc}
+	*/
+	function sql_lower_text($column_name)
+	{
+		return "LOWER(SUBSTRING($column_name, 1, DATALENGTH($column_name)))";
+	}
+
 	/**
 	* Build LIKE expression
 	* @access private