UnmanagedCallersOnlyAttribute returning non-primitive value types do not work

[jkotas]

[UnmanagedCallersOnly]
static Vector2 MyCallback() { ... }

Expected: works fine
Actual: fails

More context: #35926 (comment)

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. Please help me learn by adding exactly one area label.

[jkoritzinsky]

Since this depends on a fix for #12375 and #39294 isn't going to make .NET 5, I'm going to move this to .NET 6.

[marek-safar]

/cc @vargaz

[vargaz]

How is this going to work without marshal attributes ?