Relicensing IREE

[Geoffrey Martin-Noble]

Hi all,

To facilitate code sharing with LLVM, we have decided to relicense the IREE project under the same license, that is the Apache 2.0 License with LLVM Exceptions. Luckily our CLA makes that easy to do.

It has also come up recently that others have wanted to add their copyright attribution to the source files to recognize their contributions. To facilitate this in a scalable way, we will be changing the copyright headers to be "Copyright [YYYY] The IREE Authors" and adding an AUTHORS file to the root of the repository (as described in https://opensource.google/docs/releasing/authors/). Entities who have made significant contributions to IREE will be listed in this file and new ones can be added. To start out with, I intend to list Google LLC, Fraunhofer Institute for Material Flow and Logistics, and Nod Labs. For the initial PR, I'll just list Google and then send separate PRs for the other two to the appropriate parties so I make sure we get the wording right. Additional contributors should feel free to add themselves to the file after it is created. I'll add instructions regarding this to CONTRIBUTORS.md

I expect to make these changes next week. Please let me know if you have any concerns.

Best,

Geoffrey

[Geoffrey Martin-Noble]

The relicensing has been completed in https://github.com/google/iree/pull/6020. Apologies for the merge conflicts :-) Please use the new license header in future additions to the repo.