[BUG]: Consolidate conflicting licenses

### Is this a duplicate?

- [X] I confirmed there appear to be no [duplicate issues](https://github.com/NVIDIA/cccl/issues) for this bug and that I agree to the [Code of Conduct](CODE_OF_CONDUCT.md)

### Type of Bug

Something else

### Component

libcu++

### Describe the bug

While we have different licenses between thrust, cub and libcu++ we also have conflicting licenses within libcu++.

We should be consistent in our usage of licenses that make clear, what is code that we modified and what is code that comes directly from llvm libc++.

### How to Reproduce

Nothing, just the headers

### Expected behavior

We should either use our libcu++ header:
```
//===----------------------------------------------------------------------===//
//
// Part of libcu++, the C++ Standard Library for your entire system,
// 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
// SPDX-FileCopyrightText: Copyright (c) 2023 NVIDIA CORPORATION & AFFILIATES.
//
//===----------------------------------------------------------------------===//
```

or the llvm header:
```
// -*- C++ -*-
//===-------------------------- memory ------------------------------------===//
//
// Part of the LLVM Project, 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
//
//===----------------------------------------------------------------------===//
```
My personal proposal would be that everything that is inside libcxx remains with the llvm license, as well as everything that is not exposed through libcu++.

Any other header / test file should come with our own license header.

### Reproduction link

_No response_

### Operating System

_No response_

### nvidia-smi output

_No response_

### NVCC version

_No response_