OverloadedFunctionDefintion.Add does not guard against ambiguous overloads

[ewoutkramer]

When an OverloadedFunctionDefinition is created using the Combine factory methods, the CanCombine function is called to check that none of the overloads is ambiguous by having exactly the same signature. However, once constructed, one can use the Add method to add additional overloads, for which these checks are not done.

We should fix the Add function so it does check for uniqueness.