[C#] Fix collision of member if union name is "Value"

[tira-misu]

If an enum is called "Value" this collide with a generated member in C# source code. The generated code will not compile.

Example:

union Value {
  test1, test2
}

This will generate:

public enum Value : byte
{
  test1 = 0,
  test2 = 1,
}
 
public ValueUnion() {
  this.Type = Value.NONE;
  this.Value = null; // <- this will collide with "public enum Value"
}

This fix names the member variable "Value_" instead of "Value" if union name is "Value". This has the smallest effect on existing generated code.