[SPARK-19472][SQL] Parser should not mistake CASE WHEN(...) for a function call

## What changes were proposed in this pull request?
The SQL parser can mistake a `WHEN (...)` used in `CASE` for a function call. This happens in cases like the following:
```sql
select case when (1) + case when 1 > 0 then 1 else 0 end = 2 then 1 else 0 end
from tb
```
This PR fixes this by re-organizing the case related parsing rules.

## How was this patch tested?
Added a regression test to the `ExpressionParserSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16821 from hvanhovell/SPARK-19472.