pgsql-server: Restructure plpgsql's parsing of datatype declarations to

Log Message:
-----------
Restructure plpgsql's parsing of datatype declarations to unify the
scalar and composite (rowtype) cases a little better.  This commit is
just a code-beautification operation and shouldn't make any real
difference in behavior, but it's an important preliminary step for
trying to improve plgsql's handling of rowtypes.

Modified Files:
--------------
    pgsql-server/src/pl/plpgsql/src:
        gram.y (r1.53 -> r1.54)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/gram.y.diff?r1=1.53&r2=1.54)
        pl_comp.c (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/pl_comp.c.diff?r1=1.75&r2=1.76)
        plpgsql.h (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.45&r2=1.46)
        scan.l (r1.34 -> r1.35)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/scan.l.diff?r1=1.34&r2=1.35)