Improve support for Clang inside of UPB

[Vadmeme]

After using UPB I noticed that field accessors were not getting inlined properly using Clang. The source appears to be the UPB_FORCEINLINE macro falling back to just being static when underneath clang despite clang having full support of the required attributes.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[github-actions[bot]]

We triage inactive PRs and issues in order to make it easier to find active work. If this PR should remain active, please add a comment.

This PR is labeled inactive because the last activity was over 90 days ago. This PR will be closed and archived after 14 additional days without activity.

[Vadmeme]

Any updates on this?

[tonyliaoss]

I saw some CI failures but they look unrelated to the CL.

Just did a force-pull with Copybara and the CI tests looked good: https://github.com/protocolbuffers/protobuf/pull/19358