Add explicit layout checks to Crossgen2 (dotnet/coreclr#27054)

Add explicit layout checks to Crossgen2 (dotnet/coreclr#27054)

This change adds explicit layout checks using a loose port of the
CoreCLR algorithm i.e. creating an array representing the individual
bytes of the type layout and gradually filling it in with GC reference /
non-GC reference markers for the individual fields.

Thanks

Tomas

Commit migrated from https://github.com/dotnet/coreclr/commit/97c582975eed3260ef21e0bcc55c8c86d1e3bec8