Fix definition of promote_rule(Missing, Missing)

I don't believe this definition is ever actually used (because `promote_type(::Type{T}, ::Type{T}) where {T}` would intercept it earlier), but it is still useful for preventing the ambiguities test to complain. As such, we might as well make it correct.

base/missing.jl

@@ -31,7 +31,8 @@ for U in (:Nothing, :Missing)
  promote_rule(::Type{Union{S,$U}}, ::Type{T}) where {T,S} = Union{promote_type(T, S), $U}
  promote_rule(::Type{Any}, ::Type{$U}) = Any
  promote_rule(::Type{$U}, ::Type{Any}) = Any
- promote_rule(::Type{$U}, ::Type{$U}) = U
+ # This definition is never actually used, but disambiguates the above definitions
+ promote_rule(::Type{$U}, ::Type{$U}) = $U
  end
 end
 promote_rule(::Type{Union{Nothing, Missing}}, ::Type{Any}) = Any