If you take a look at the CONTRIBUTING.md, Mold has taken an approach that is pretty novel to me. Instead of requiring a CLA to allow for relicensing, all patches are required to be released as dual-license AGPLv3 and MIT. I personally love this approach, but it could make it difficult to relicense to anything but MIT.
MIT is a very permissive and compatible with lots of other open-source licenses. For example, you can sublicense MIT-licensed code under GPL. Not sure if MIT is compatible with the LLVM license, though.
