merging duplicate definitions of adjust_relid_set

While reviewing Ashutosh Bapat's partitionwise join code, I noticed
he'd run up against the problem that adjust_relid_set() is defined as
static in two different source files, and he wanted to call it from a
third file. I didn't much like his solution to that problem, which
was to rename one of them and make that definition non-static; I think
it would be better to keep the existing name and stop defining it in
multiple places. However, I discovered that there wasn't really an
obviously-good place to put the function; neither prepunion.c nor
rewriteManip.c, the two files that contain static versions as of now,
seem like an appropriate place from which to expose it, and I didn't
find anything else that I was wildly in love with, either. The
attached patch puts it in var.c, because it didn't look horrible and I
thought it wasn't worth creating a new file just for this.

Objections, better ideas?

--
Robert Haas
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