Non-blittable struct with explicit layout has wrong managed size

[jkotas]

Context #100206 (comment)

Repro

using System.Runtime.InteropServices;
using System.Runtime.CompilerServices;

Console.WriteLine(Unsafe.SizeOf<MyStruct>());

[StructLayout(LayoutKind.Explicit, Size = 9)]
struct MyStruct
{
    [FieldOffset(0)]
    public byte[] a;
    [FieldOffset(8)]
    public byte b;
}

Expected result (on x64)

16

Actual result

9

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Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
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[MichalStrehovsky]

The layout logic is shared with crossgen2; this also reproes with ReadyToRun.

The type layout has been owned/worked on by the crossgen2 team since it needs to be bug-for-bug compatible with CoreCLR VM type system for ReadyToRun to work. Moving to crossgen2 area path.

[steveisok]

/cc @kg