[SPARK-15276][SQL] CREATE TABLE with LOCATION should imply EXTERNAL

## What changes were proposed in this pull request?

Before:
```sql
-- uses that location but issues a warning
CREATE TABLE my_tab LOCATION /some/path
-- deletes any existing data in the specified location
DROP TABLE my_tab
```

After:
```sql
-- uses that location but creates an EXTERNAL table instead
CREATE TABLE my_tab LOCATION /some/path
-- does not delete the data at /some/path
DROP TABLE my_tab
```

This patch essentially makes the `EXTERNAL` field optional. This is related to #13032.

## How was this patch tested?

New test in `DDLCommandSuite`.

Author: Andrew Or <andrew@databricks.com>

Closes #13060 from andrewor14/location-implies-external.