[cxx-interop] calls to __swift_interopStaticCast in synthesized base class member accessors should not copy C++ type by value

Right now, when Swift compiler synthesizes a call to `__swift_interopStaticCast` when creating a thunk for imported C++ type that calls a const base class  method (non-mutating), it ends up passing the C++ record by value, thus copying the given value. This should be avoided, we should pass them by const ref or pointer, like we do when invoking mutating methods.