Make clang's rewrite engine a core feature

The rewrite facility's footprint is small so it's not worth going to these
lengths to support disabling at configure time, particularly since key compiler
features now depend on it.

Meanwhile the Objective-C rewriters have been moved under the
ENABLE_CLANG_ARCMT umbrella for now as they're comparatively heavy and still
potentially worth excluding from lightweight builds.

Tests are now passing with any combination of feature flags. The flags
historically haven't been tested by LLVM's build servers so caveat emptor.