License header incorrectly states that libcu++ is a part of the CUDA toolkit and does not assert copyright

[brycelelbach]

Our current license header says:

//===----------------------------------------------------------------------===//
//
// Part of the CUDA Toolkit, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

libcu++ is not a part of the CUDA Toolkit. It is an independent open source project, developed on GitHub, which is released with the NVIDIA HPC SDK and the CUDA Toolkit. The license header needs to be updated to reflect this reality.

The header also needs to say "Copyright (c) YYYY NVIDIA Corporation" to assert our copyright.