[SPARK-20688][SQL] correctly check analysis for scalar sub-queries

## What changes were proposed in this pull request?

In `CheckAnalysis`, we should call `checkAnalysis` for `ScalarSubquery` at the beginning, as later we will call `plan.output` which is invalid if `plan` is not resolved.

## How was this patch tested?

new regression test

Author: Wenchen Fan <wenchen@databricks.com>

Closes #17930 from cloud-fan/tmp.