detect mismatch in generics when redeclaring a function

[hishamhm]

This should fail, but it succeeds silently in next and crashes the compiler in 0.15.x.

local type List2 = record<T>
   new: function<U>(initialItems: {T}, u: U): List2<T>
end

function List2.new<U>(initialItems: {T}, u: U): List2<U> -- mismatched return type
end

local type Fruit2 = enum
   "apple"
   "peach"
   "banana"
end

local type L2 = List2<Fruit2>
local lunchbox = L2.new({"apple", "peach"}, true)