Implement PREPARE AS statement for ECPG.

Michael Meskes <meskes@postgresql.org>

Commit: a1dc6ab465986a62b308dd1bb8da316b5ed9685a
Author: Michael Meskes <meskes@postgresql.org>
Date: 2019-05-22T02:58:29Z
Releases: 12.0
Implement PREPARE AS statement for ECPG.

Besides implementing the new statement this change fix some issues with the
parsing of PREPARE and EXECUTE statements. The different forms of these
statements are now all handled in a ujnified way.

Author: Matsumura-san <matsumura.ryo@jp.fujitsu.com>

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