[SPARK-15257][SQL] Require CREATE EXTERNAL TABLE to specify LOCATION

## What changes were proposed in this pull request?

Before:
```sql
-- uses warehouse dir anyway
CREATE EXTERNAL TABLE my_tab
-- doesn't actually delete the data
DROP TABLE my_tab
```
After:
```sql
-- no location is provided, throws exception
CREATE EXTERNAL TABLE my_tab
-- creates an external table using that location
CREATE EXTERNAL TABLE my_tab LOCATION '/path/to/something'
-- doesn't delete the data, which is expected
DROP TABLE my_tab
```

## How was this patch tested?

New test in `DDLCommandSuite`

Author: Andrew Or <andrew@databricks.com>

Closes #13032 from andrewor14/create-external-table-location.