Support csharp_partial for unions

[inikulshin]

E.g.

union ArgumentValue (csharp_partial) {
  ArgumentValueString,
  ArgumentValueArrayByte,
  ArgumentValueNumber,
  ArgumentValueBoolean,
  ArgumentValueList
}

Firstly it's just a better design, to allow csharp_partial for all generated classes, without exception.

But it may be useful for defining more brief constructors:

public ArgumentValueUnion(string value)
{
    Value = ArgumentValueUnion.FromArgumentValueString(new ArgumentValueStringT { Value = value });
}

or overriding ToString():

public override string ToString()
{
    switch ...
}