Bogus rte->relkind for EXCLUDED pseudo-relation

In the wake of b23cd185f (pushed just now), I tried adding Asserts
to rewriteHandler.c that relkinds in RTEs don't change, as attached.
This blew up the regression tests immediately. On investigation,
I find that

(1) ON CONFLICT's EXCLUDED pseudo-relation is assigned
rte->relkind = RELKIND_COMPOSITE, a rather horrid hack
installed by commit ad2278379.

(2) If a stored rule involves ON CONFLICT, then while loading
the rule AcquireRewriteLocks overwrites that with the actual
relkind, ie RELKIND_RELATION. Or it did without the
attached Assert, anyway.

It appears to me that this means whatever safeguards are created
by the use of RELKIND_COMPOSITE will fail to apply in a rule.
Maybe that's okay because the relevant behaviors only occur at
parse time not rewrite/planning/execution, but even to write that
is to doubt how reliable and future-proof the assumption is.

I'm inclined to think we'd be well advised to undo that aspect of
ad2278379 and solve it some other way. Maybe a new RTEKind would
be a better idea. Alternatively, we could drop rewriteHandler.c's
attempts to update relkind. Theoretically that's safe now, but
I hadn't quite wanted to just pull that trigger right away ...

regards, tom lane