Centralise set<Transition*> -> Constant* logic.

Previously, we were duplicating the code to convert a set of TESLA transitions into LLVM constants. This caused headaches when attempting to modify other things, so put this logic in one place.
Don't pass CFLAGS to clang-as-linker.

When using the Clang driver as a linker frontend, don't pass CFLAGS that it doesn't care about (and that cause warnings).
Special tags should go next to event description.

Tags like <<init>> or <<cleanup>> should go next to the event description, not the destination node number.
Only take one transition.

When presented with a menu of possible transitions to take, we should only take one of them.
Improvements to TESLA runtime debug switching.
We should never see multiple <<init>> transitions.