[llvm] workflows: Factor out artifact attestation and upload into a composite action (PR #169621)

[Tom Stellard via llvm-commits]

tstellar wrote:

> I'm assuming there is a plan to migrate other workflows that upload release artifacts to use this action as well?

Yes, that's right.  I have a draft PR that uses this for Windows release builds, and am planning to switch over the other release builds too.

https://github.com/llvm/llvm-project/pull/169621