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/backend/nodes/readfuncs.c

@@ -1388,6 +1388,7 @@ _readDefElem(void)
 	READ_STRING_FIELD(defname);
 	READ_NODE_FIELD(arg);
 	READ_ENUM_FIELD(defaction, DefElemAction);
+	READ_LOCATION_FIELD(location);
 
 	READ_DONE();
 }