pgsql: Instead of trying (and failing) to allow <<label>> at the end of

Log Message:
-----------
Instead of trying (and failing) to allow <<label>> at the end of a DECLARE
section, throw an error message saying explicitly that the label must go
before DECLARE. Per investigation of a recent pgsql-novice question,
this code did not work as intended in any modern PG version, maybe not ever.
Allowing such a thing would only create ambiguity anyway, so it seems better
to remove it than fix it.

Modified Files:
--------------
pgsql/src/pl/plpgsql/src:
gram.y (r1.141 -> r1.142)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.141&r2=1.142)