Clean up plpgsql grammar to make sure that check_assignable() is applied

Clean up plpgsql grammar to make sure that check_assignable() is applied
consistently.  This is mostly cosmetic right at the moment because
check_assignable() does nothing for ROW or RECORD datums, but that might
not always be so.  This also syncs several different places that read
INTO target lists.  They're just enough different that it seems
impractical to factor them into a single routine, but they surely
should be the same as much as possible.