Per spec we ought to apply select_common_collation() across the expressions
in each column of the VALUES table.  The original coding was just taking
the first row and assuming it was representative.

This patch adds a field to struct RangeTblEntry to carry the resolved
collations, so initdb is forced for changes in stored rule representation.

If the referencing and referenced columns have different collations,
the parser will be unable to resolve which collation to use unless it's
helped out in this way.  The effects are sometimes masked, if we end up
using a non-collation-sensitive plan; but if we do use a mergejoin
we'll see a failure, as recently noted by Robert Haas.

The SQL spec states that the referenced column's collation should be used
to resolve RI checks, so that's what we do.  Note however that we currently
don't append a COLLATE clause when writing a query that examines only the
referencing column.  If we ever support collations that have varying
notions of equality, that will have to be changed.  For the moment, though,
it's preferable to leave it off so that we can use a normal index on the
referencing column.

Remove crude hack that tried to propagate collation through a
function-returning-record, ie, from the function's arguments to individual
fields selected from its result record.  That is just plain inconsistent,
because the function result is composite and cannot have a collation;
and there's no hope of making this kind of action-at-a-distance work
consistently.  Adjust regression test cases that expected this to happen.

Meanwhile, the behavior of casting to a domain with a declared collation
stays the same as it was, since that seemed to be the consensus.

Ensure that COLLATE at the top level of an index expression is treated the
same as a grammatically separate COLLATE.  Fix bogus reverse-parsing logic
in pg_get_indexdef.

This restores a parse error that was thrown (though only in the ORDER BY
case) by the original collation patch.  I had removed it in my recent
revisions because it was thrown at a place where collations now haven't
been computed yet; but I thought of another way to handle it.

Throwing the error at parse time, rather than leaving it to be done at
runtime, is good because a syntax error pointer is helpful for localizing
the problem.  We can reasonably assume that the comparison function for a
collatable datatype will complain if it doesn't have a collation to use.
Now the planner might choose to implement GROUP or DISTINCT via hashing,
in which case no runtime error would actually occur, but it seems better
to throw error consistently rather than let the error depend on what the
planner chooses to do.  Another possible objection is that the user might
specify a nondefault sort operator that doesn't care about collation
... but that's surely an uncommon usage, and it wouldn't hurt him to throw
in a COLLATE clause anyway.  This change also makes the ORDER BY/GROUP
BY/DISTINCT case more consistent with the UNION/INTERSECT/EXCEPT case,
which was already coded to throw this error even though the same objections
could be raised there.

There's a lot we can't test very well without platform dependencies,
but the C/POSIX collations should now work the same way everywhere.