Extend Task.FromResult default task optimization to 16 byte unmanaged types (#87541)

Extend Task.FromResult default task optimization to 16 byte unmanaged types (#87541)

* Extend Task.FromResult default task optimization to 16 byte unmanaged types

For years, async methods / Task.FromResult has cached a `Task<T>` for `default(T)`, however it was only used when the result value was null.  Earlier in this release we extended that optimization to also use the default task when the value was 1, 2, 4, or 8 bytes (and not a reference type).  This extends that further to also handle types that are 16 bytes, so as to include types like Decimal, Guid, Int128, and DateTimeOffset.

* Fix tests to accomodate additional use of default cached task