Updates CompactUnwindTraits_MachO_arm64 and CompactUnwindTraits_MachO_x86_64 encodingCanBeMerged methods to use switch statements that clearly list mergeable encodings, and have a default "false" case. Since the new scheme explicitly covers DWARF modes (always non-mergeable), this patch removes the separate DWARF mode check from mergeRecords in CompactUnwindSupport.h.