Improve pg_dump/pg_restore --create --if-exists logic.

Improve pg_dump/pg_restore --create --if-exists logic.

Teach it not to complain if the dropStmt attached to an archive entry
is actually spelled CREATE OR REPLACE VIEW, since that will happen due to
an upcoming bug fix.  Also, if it doesn't recognize a dropStmt, have it
print a WARNING and then emit the dropStmt unmodified.  That seems like a
much saner behavior than Assert'ing or dumping core due to a null-pointer
dereference, which is what would happen before :-(.

Back-patch to 9.4 where this option was introduced.

Discussion: <19092.1479325184@sss.pgh.pa.us>