Add location field to DefElem

Add a location field to the DefElem struct, used to parse many utility
commands.  Update various error messages to supply error position
information.

To propogate the error position information in a more systematic way,
create a ParseState in standard_ProcessUtility() and pass that to
interested functions implementing the utility commands.  This seems
better than passing the query string and then reassembling a parse state
ad hoc, which violates the encapsulation of the ParseState type.

Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>

src/include/nodes/parsenodes.h

@@ -666,6 +666,7 @@ typedef struct DefElem
 	char	   *defname;
 	Node	   *arg;			/* a (Value *) or a (TypeName *) */
 	DefElemAction defaction;	/* unspecified action, or SET/ADD/DROP */
+	int			location;		/* token location, or -1 if unknown */
 } DefElem;
 
 /*