Upgrade to LLVM 20 for better static analysis

The PR associated https://github.com/libhal/libhal/actions/runs/17277387182/job/49037497880?pr=162 builds but fails testing. It gets a stack use after scope error due to the temporary array having its address captured, then that object being dropped. In LLVM 20 (tested) this becomes a compile time error that explains exactly what has gone wrong with the operations.