Re: RangeTblEntry.joinaliasvars representation change

I wrote:
> After some reflection I think that the best fix is to redefine
> AcquireRewriteLocks' processing of dropped columns so that it puts an
> actual null pointer, not a consed-up Const, into the joinaliasvars list
> item.

Here's a complete patch along that line. Possibly worthy of note is
that I chose to keep expandRTE's API the same as before, ie, it returns
a NULL Const for a dropped column (when include_dropped is true). I had
intended to change it to return a null pointer to match the change in
the underlying data structure, but found that most of the callers
passing include_dropped = true need the Consts, because they're going to
construct a RowExpr that has to have null constants for the dropped
columns.

In HEAD, I'm a bit tempted to move strip_implicit_coercions into
nodes/nodeFuncs.c, so that we don't have the ugliness of the parser
calling an optimizer subroutine. But I propose applying this to back
branches as-is.

regards, tom lane