Make Reinterop skip attributes with 'Ignore' attribute

[j9liu]

Description

This PR supports #656 and enables the use of Unity's Color32 struct with Reinterop.

As @kring discovered, Color32 has an int field that doesn't actually contribute to the struct's size. Reinterop was turning this into an actual int field, preventing the actual r,g etc. bytes from storing value.

[StructLayout(LayoutKind.Explicit)]
[UsedByNativeCode]
public struct Color32 : IEquatable<Color32>, IFormattable
{
    [FieldOffset(0)]
    [Ignore(DoesNotContributeToSize = true)]
    private int rgba;

    //
    // Summary:
    //     Red component of the color.
    [FieldOffset(0)]
    public byte r;

    //
    // Summary:
    //     Green component of the color.
    [FieldOffset(1)]
    public byte g;

    //
    // Summary:
    //     Blue component of the color.
    [FieldOffset(2)]
    public byte b;

    //
    // Summary:
    //     Alpha component of the color.
    [FieldOffset(3)]
    public byte a;
 

The easiest way to avoid this is to tell Reinterop to ignore anything with that Ignore attribute. It hasn't resulted in any surprises in any other generated classes; I checked with diff tools.

Testing plan

1. Rebuild Reinterop using dotnet publish Reinterop~ -o . before opening Unity.
2. Open Unity, then add a line like Color32 color = new Color32(0, 0, 0, 1) to Reinterop
3. Wait for Reinterop to complete, then navigate to native~/generated-Editor/include/DotNet/UnityEngine/Color32.h.
4. Confirm that the uint8 r, g, b, and a fields show up, but not an int32 rgba field.

[david-lively]

Short and simple. Looks good to me.