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.
d16413ad22