Remove StructLayout LayoutKind.Auto from DateTime

[Tornhoof]

As suggested by @tannergooding in dotnet/csharplang#206 (comment) the [StructLayout(LayoutKind.Auto)] on DateTime is not necessary and might prevent the wider adoption of the Initial blittable proposal. DateTime only contains a single private long field.

[gkhanna79]

CC @danmosemsft

[ufcpp]

How about ValueTuple<...>?
I expect ValueTupeles are blittable if its all type arguments are blittable. However, They are not so because their LayoutKind is Auto.

[tannergooding]

@ufcpp, while useful. I'm not sure if removing it from ValueTuple is possible.

In the case of DateTime, it only contains a single field (a long), so it will always have the same layout.

With ValueTuple, there are a multitude of fields, so ValueTuple<short, long, short, int> would be 24 bytes for Sequential Layout and 16 bytes for Auto layout. On top of that, ValueTuple suffers from the same issue as DateTimeOffset in that Auto makes the layout change with regards to surrounding fields, not just with regards to the fields in the struct itself: https:/dotnet/coreclr/issues/10449#issuecomment-292577303