Fix imported constructors and instance methods

[hermann-noll]

Constructors were supported when they had bodies and were generated as actual C# constructors, but trying to import them produced invalid C#, e.g:

    public partial struct MyStruct
    {
        [DllImport("", CallingConvention = CallingConvention.ThisCall, EntryPoint = "...", ExactSpelling = true)]
        public static extern MyStruct();
    }

So three things had to be fixed:

• The name has to be remapped to not conflict with C# constructors (static or not)
  Also copy/move constructors will conflict with each other thus they get different names
• A return type has to be added (Clang said void although it might also be a pointer to the struct?)
• A pThis parameter has to be added.
  This point applied to all imported instance methods.

Here is an example of the result of this PR:

    public partial struct MyStruct
    {
        [DllImport("", CallingConvention = CallingConvention.ThisCall, EntryPoint = "...", ExactSpelling = true)]
        public static extern unsafe void Constructor(MyStruct* pThis);

        [DllImport("", CallingConvention = CallingConvention.ThisCall, EntryPoint = "...", ExactSpelling = true)]
        public static extern unsafe void CopyConstructor(MyStruct* pThis, [NativeTypeName("const MyStruct &")] MyStruct* other);

        [DllImport("", CallingConvention = CallingConvention.ThisCall, EntryPoint = "...", ExactSpelling=true)]
        public static extern unsafe void MoveConstructor(MyStruct* pThis, [NativeTypeName("MyStruct &&")] MyStruct* other);

        [DllImport("", CallingConvention = CallingConvention.ThisCall, EntryPoint = "...", ExactSpelling = true)]
        public static extern unsafe void MyMethod(MyStruct* pThis);
    }