In 15.10, llvm-runtime, and llvm-dev resolve to llvm-3.6-runtime, and llvm-3.6-dev.
Nothing is put into the alternative system.
So if llvm-3.7-runtime, and llvm-3.7 dev are installed, there is really no way to pick a default to switch between revisions.
If the installations at least added everything to alternatives, with the latest version given the highest priority, there would be a way to upgrade automatically, and manually select a desired version.
In 15.10, llvm-runtime, and llvm-dev resolve to llvm-3.6-runtime, and llvm-3.6-dev.
Nothing is put into the alternative system.
So if llvm-3.7-runtime, and llvm-3.7 dev are installed, there is really no way to pick a default to switch between revisions.
If the installations at least added everything to alternatives, with the latest version given the highest priority, there would be a way to upgrade automatically, and manually select a desired version.