[C# 8.0] 12.6.6.1 should not require a defensive copy to invoke a `readonly` member

[BillWagner]

To fix #894, a defensive copy is required to an in parameter of a non-readonly struct type to invoke a member.

C# 8 adds readonly members to non-readonly structs. A defensive copy isn't required to invoke a readonly member.

See #927 (desccription)

[BillWagner]

See #673 This change should be added to that PR.

[jskeet]

We suspect this may be in #673; @BillWagner to just check and close if so.