xcframework Headers root restored for cached static-objc-behind-dynamic deps

Static Objective-C xcframeworks that sit behind a dynamic framework in the binary cache now get their matching slice’s Headers root added to HEADER_SEARCH_PATHS, on top of the module map’s own directory. This resolves the shape of dependency that ARCore (and similarly laid-out Google SDK xcframeworks) hit, where module-map-less static-objc headers were no longer reachable for consumers.