Overload resolution does not eliminate incompatible Nullable candidates

[NinoFloris]

Found in #13673

type M() =
    static member A(n: System.Nullable<char>) = printfn "1"
    static member A(n: int) = printfn "4"
let test(x: 'T) =
    M.A(x)

If we know nothing we should prefer int, 'T should always be better than Nullable<'T>. Today we produce an ambiguity error.

https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AbEAzAzmgFxAEsMAfAgTwAcYACAWQAoBKOgXgFgAoOvu3AQCGBYmDoBbGBOAwodAIJMAdiDoBlSoOkA6AHIBXDBiHAMMADxgAFkKgA+NuzrUoxZQWzK6AIgCMPjz8AsKi4lIycooqau4ETi5uHl6+KIHc5gR0BDCCTAhqAOQAKk5B/Aw6SggsQA===