Improve handling of inherited GENERATED expressions.

Improve handling of inherited GENERATED expressions.

In both partitioning and traditional inheritance, require child
columns to be GENERATED if and only if their parent(s) are.
Formerly we allowed the case of an inherited column being
GENERATED when its parent isn't, but that results in inconsistent
behavior: the column can be directly updated through an UPDATE
on the parent table, leading to it containing a user-supplied
value that might not match the generation expression.  This also
fixes an oversight that we enforced partition-key-columns-can't-
be-GENERATED against parent tables, but not against child tables
that were dynamically attached to them.

Also, remove the restriction that the child's generation expression
be equivalent to the parent's.  In the wake of commit 3f7836ff6,
there doesn't seem to be any reason that we need that restriction,
since generation expressions are always computed per-table anyway.
By removing this, we can also allow a child to merge multiple
inheritance parents with inconsistent generation expressions, by
overriding them with its own expression, much as we've long allowed
for DEFAULT expressions.

Since we're rejecting a case that we used to accept, this doesn't
seem like a back-patchable change.  Given the lack of field
complaints about the inconsistent behavior, it's likely that no
one is doing this anyway, but we won't change it in minor releases.

Amit Langote and Tom Lane

Discussion: https://postgr.es/m/2793383.1672944799@sss.pgh.pa.us