pgsql: Fix several DDL issues of generated columns versus inheritance

Fix several DDL issues of generated columns versus inheritance

Several combinations of generated columns and inheritance in CREATE
TABLE were not handled correctly.  Specifically:

- Disallow a child column specifying a generation expression if the
  parent column is a generated column.  The child column definition
  must be unadorned and the parent column's generation expression will
  be copied.

- Prohibit a child column of a generated parent column specifying
  default values or identity.

- Allow a child column of a not-generated parent column specifying
  itself as a generated column.  This previously did not work, but it
  was possible to arrive at the state via other means (involving ALTER
  TABLE), so it seems sensible to support it.

Add tests for each case.  Also add documentation about the rules
involving generated columns and inheritance.

Discussion:
    https://www.postgresql.org/message-id/flat/15830.1575468847%40sss.pgh.pa.us
    https://www.postgresql.org/message-id/flat/2678bad1-048f-519a-ef24-b12962f41807%40enterprisedb.com
    https://www.postgresql.org/message-id/flat/CAJvUf_u4h0DxkCMCeEKAWCuzGUTnDP-G5iVmSwxLQSXn0_FWNQ%40mail.gmail.com

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/14751c340754af9f906a893eb87a894dea3adbc9

Modified Files
--------------
doc/src/sgml/ddl.sgml                   | 26 ++++++++++++++
src/backend/commands/tablecmds.c        | 61 ++++++++++++++++++++++++++++++---
src/test/regress/expected/generated.out | 44 +++++++++++++++++++++++-
src/test/regress/sql/generated.sql      | 22 +++++++++++-
4 files changed, 146 insertions(+), 7 deletions(-)