Prohibit locking clauses on GRAPH_TABLE

Prohibit locking clauses on GRAPH_TABLE

Specifying a locking clause (FOR UPDATE/SHARE) that names a
GRAPH_TABLE alias currently results in an unhelpful "unrecognized RTE
type: 8" error.  This commit explicitly prohibits specifying a locking
clause on a GRAPH_TABLE alias, raising a more user-friendly error
instead.

(Locking clause support for GRAPH_TABLE could be added as a separate
feature in the future.)

Author: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAHg%2BQDcE9wp6nOEC3SCRQ90nrCO%3DQF%2BOZq1MG8Qc6hnusmogqw%40mail.gmail.com