Given a particular set of arguments the function is applied to, returns the correct overloaded form of the function.
The method adds an item to conversions
every time it performs an implicit type-conversion. If there are several candidate functions with the same signature, the validator will choose the one which used the fewest implicit conversions.
@param args Expressions which this function call is applied to.
@param validator Validator
@param conversions List of implicit conversions performed (out)
@return The function definition which matches these arguments, or nullif no function definition that this resolver knows about matches.