pgsql: Fix ALTER TABLE ADD COLUMN so that constraints of domain types

Log Message:
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Fix ALTER TABLE ADD COLUMN so that constraints of domain types are
enforced properly when there is no explicit default value for the new
column. Per report from Craig Perras.

Tags:
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REL8_0_STABLE

Modified Files:
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pgsql/src/backend/commands:
tablecmds.c (r1.142 -> r1.142.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.142&r2=1.142.4.1)
typecmds.c (r1.65 -> r1.65.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c.diff?r1=1.65&r2=1.65.4.1)