Added a pass to allow struct to struct marshalling if possible

[genuinelucifer]

@tritao @ddobrev Please review. It is based on one of my earlier pulls and the first commit is from that only.
This pull tries to address #500 .

[genuinelucifer]

As far as I have checked it, now it has to have a check that if there is an explicitly defined constructor/destructor [like in 'TestDestructor' in CSharpTemp.h] then too it must NOT be marshalled as a struct.

[tritao]

Extra newline here.

[genuinelucifer]

will remove.

[genuinelucifer]

if we have code like:

struct RefType
{
protected:
    int n;
public:
    virtual int getN() { return n; }
};

struct CS_VALUE_TYPE ValType : public RefType
{
};

Then uncompilable code was generated. We simply copy members from the base into the derived type if the derived type is a value type. But value types can't have protected &/or virtual members. So, i just wanted to fix that.

[ddobrev]

@genuinelucifer your answer shows that you've ignored part of @tritao 's question. He's asked you if the code is STILL necessary when you consider our new discovery - value types cannot derive from ref types in C# either.

[genuinelucifer]

@ddobrev @tritao Sorry. I was leaving for some place and wrote that in a hurry.
Since it is not supported so i don't think there is any need of keeping it. I will remove it.