Fix method ambiguity for Base.TwicePrecision

[devmotion]

The PR fixes the following error on the master branch:

julia> using ForwardDiff

julia> ForwardDiff.derivative(x -> sum(1 .+ x .* (0:0.1:1)), 1)
ERROR: MethodError: ForwardDiff.Dual{ForwardDiff.Tag{var"#3#4", Int64}, Float64, 1}(::Base.TwicePrecision{ForwardDiff.Dual{ForwardDiff.Tag{var"#3#4", Int64}, Float64, 1}}) is ambiguous.

Candidates:
  ForwardDiff.Dual{T, V, N}(x) where {T, V, N}
    @ ForwardDiff ~/.julia/packages/ForwardDiff/L9SLV/src/dual.jl:77
  (::Type{T})(x::Base.TwicePrecision) where T<:Number
    @ Base twiceprecision.jl:265

Possible fix, define
  ForwardDiff.Dual{T, V, N}(::Base.TwicePrecision) where {N, V, T}

Stacktrace:
 [1] broadcasted(::Base.Broadcast.DefaultArrayStyle{…}, ::typeof(+), x::Int64, r::StepRangeLen{…})
   @ Base.Broadcast ./broadcast.jl:1120
 [2] broadcasted
   @ ./broadcast.jl:1326 [inlined]
 [3] (::var"#3#4")(x::ForwardDiff.Dual{ForwardDiff.Tag{var"#3#4", Int64}, Int64, 1})
   @ Main ./REPL[4]:1
 [4] derivative(f::var"#3#4", x::Int64)
   @ ForwardDiff ~/.julia/packages/ForwardDiff/L9SLV/src/derivative.jl:14
 [5] top-level scope
   @ REPL[4]:1
Some type information was truncated. Use `show(err)` to see complete types.