[RFC] Introduce convergence control intrinsics

This is a reboot of the original design and implementation by
Nicolai Haehnle <nicolai.haehnle@amd.com>:
https://reviews.llvm.org/D85603

This change also obsoletes an earlier attempt at restarting the work on
convergence tokens:
https://reviews.llvm.org/D104504

Changes relative to D85603:

 1. Clean up the definition of a "convergent operation", a convergent
    call and convergent function.
 2. Clean up the relationship between dynamic instances, sets of threads and
    convergence tokens.
 3. Redistribute the formal rules into the definitions of the convergence
    intrinsics.
 4. Expand on the semantics of entering a function from outside LLVM,
    and the environment-defined outcome of the entry intrinsic.
 5. Replace the term "cycle" with "closed path". The static rules are defined
    in terms of closed paths, and then a relation is established with cycles.
 6. Specify that if a function contains a controlled convergent operation, then
    all converg...