Mark `MemmappedTensorAllocator` as returning opaque handle.

This allocator is used for `ImmutableConstantOp` and it returns a handle to the contents of a memory mapped file which is supposed to represent a tensor.

For tensors of complex types (resources, variables and strings), allocators which are not marked as returning opaque handles will call placement new to initialize each element. This means writing to the buffer. However, in our case, the buffer is immutable and already contains the tensor data. Hence, writing to it is both destructive and causes a crash.

PiperOrigin-RevId: 345786451
Change-Id: I46369c50fa60b3431709ffe068a728d3061f49c4