[SPARK-13540][SQL] Supports using nested classes within Scala objects as Dataset element type

## What changes were proposed in this pull request?

Nested classes defined within Scala objects are translated into Java static nested classes. Unlike inner classes, they don't need outer scopes. But the analyzer still thinks that an outer scope is required.

This PR fixes this issue simply by checking whether a nested class is static before looking up its outer scope.

## How was this patch tested?

A test case is added to `DatasetSuite`. It checks contents of a Dataset whose element type is a nested class declared in a Scala object.

Author: Cheng Lian <lian@databricks.com>

Closes #11421 from liancheng/spark-13540-object-as-outer-scope.