Fix crasher in icode due to symbol mismatches in lifted methods

I haven't tracked down where the parameter symbols of the
DefDef's info diverge from from those of the parameter ValDefs,
but for now I've favoured the latter when constructing the
new ValDef.

The following is logged:

  [async]  Warning: inconsistent parameter symbols on lifted method.
    - vparamss.map(_.symbol) = List(List(value m#5730))
    - dd.symbol.paramms      = List(List(value m#5731))}

Review by @phaller