Don't apply --with-packing overrides to fixed buffer types

[DaZombieKiller]

Due to an oversight in #405, --with-packing on MyStruct will also apply to any nested fixed buffer types generated for its fields.

For example, the following C structure:

struct MyStruct
{
    size_t FixedBuffer[1];
};

Will currently generate the following C# code (latest-codegen):

using System;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.InteropServices;

namespace ClangSharp.Test
{
    [StructLayout(LayoutKind.Sequential, Pack = CustomPackValue)]
    public partial struct MyStruct
    {
        [NativeTypeName("size_t[1]")]
        public _FixedBuffer_e__FixedBuffer FixedBuffer;

        [StructLayout(LayoutKind.Sequential, Pack = CustomPackValue)]
        public partial struct _FixedBuffer_e__FixedBuffer
        {
            public nuint e0;

            [UnscopedRef]
            public ref nuint this[int index]
            {
                get
                {
                    return ref AsSpan(int.MaxValue)[index];
                }
            }

            [UnscopedRef]
            public Span<nuint> AsSpan(int length) => MemoryMarshal.CreateSpan(ref e0, length);
        }
    }
}

This issue does not apply to fixed buffers of types supported naturally by C#, only those that require ClangSharp to generate an _e__FixedBuffer type.

Tests are covered in #406.