Parser: use left-recursion in apats and argpats

The following discussion from !11109 (closed) should be addressed:

• !11109 (comment 521826)

  Happy works better with left-recursive grammars, so I expected to see

  argpats :: { [LArgPat GhcPs] }  -- reversed
          : argpats argpat { $2 : $1 }
          |                { [] }

  But then I noticed that apats has the same problem, and you probably simply used it as a reference.

  We should fix this at some point, but maybe not in this patch.