Code review for protransform patches.

Fix loss of previous expression-simplification work when a transform
function fires: we must not simply revert to untransformed input tree.
Instead build a dummy FuncExpr node to pass to the transform function.
This has the additional advantage of providing a simpler, more uniform
API for transform functions.

Move documentation to a somewhat less buried spot, relocate some
poorly-placed code, be more wary of null constants and invalid typmod
values, add an opr_sanity check on protransform function signatures,
and some other minor cosmetic adjustments.

Note: although this patch touches pg_proc.h, no need for catversion
bump, because the changes are cosmetic and don't actually change the
intended catalog contents.

src/backend/parser/parse_clause.c

@@ -2271,25 +2271,3 @@ transformFrameOffset(ParseState *pstate, int frameOptions, Node *clause)
 
 	return node;
 }
-
-/*
- * relabel_to_typmod
- *		Add a RelabelType node that changes just the typmod, and remove all
- *		now-superfluous RelabelType nodes beneath it.
- */
-Node *
-relabel_to_typmod(Node *expr, int32 typmod)
-{
-	Oid			type = exprType(expr);
-	Oid			coll = exprCollation(expr);
-
-	/*
-	 * Strip any existing RelabelType, then add one. This is to preserve the
-	 * invariant of no redundant RelabelTypes.
-	 */
-	while (IsA(expr, RelabelType))
-		expr = (Node *) ((RelabelType *) expr)->arg;
-
-	return (Node *) makeRelabelType((Expr *) expr, type, typmod, coll,
-									COERCE_DONTCARE);
-}