Use a private driver registry for const eval. (#7979)
Use a private driver registry for const eval. (#7979)

Avoids conflicts when used from tools that have other uses for the process global registry.
google/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,google/iree