Pick a default for LLVM_DISTRO_NAME so it works when the script is run standalone when not on GitHub Actions CI.
Switch the logic about file location to enumerate only the couple of old versions that uses the old download location, rather than trying to enumerate an ever-growing set of modern releases that use the current download location.
Comment o say that when LLVM 10+ is the floor, the old case can go away entirely.
Also bump the CI "bleeding edge" test to use LLVM 15.0.6.
Also bump the CI "bleeding edge" test to use LLVM 15.0.6.