## What changes were proposed in this pull request?
Currently we use `CreateViewCommand` to implement ALTER VIEW AS, which has 3 bugs:
1. SPARK-17180: ALTER VIEW AS should alter temp view if view name has no database part and temp view exists
2. SPARK-17309: ALTER VIEW AS should issue exception if view does not exist.
3. SPARK-17323: ALTER VIEW AS should keep the previous table properties, comment, create_time, etc.
The root cause is, ALTER VIEW AS is quite different from CREATE VIEW, we need different code path to handle them. However, in `CreateViewCommand`, there is no way to distinguish ALTER VIEW AS and CREATE VIEW, we have to introduce extra flag. But instead of doing this, I think a more natural way is to separate the ALTER VIEW AS logic into a new command.
## How was this patch tested?
new tests in SQLViewSuite
Author: Wenchen Fan <wenchen@databricks.com>
Closes#14874 from cloud-fan/minor4.
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