[lld-macho] create __TEXT,__unwind_info from __LD,__compact_unwind

Digest the input `__LD,__compact_unwind` and produce the output `__TEXT,__unwind_info`. This is the initial commit with the major functionality.

Successor commits will add handling for ...
* `__TEXT,__eh_frame`
* personalities & LSDA
* `-r` pass-through

Differential Revision: https://reviews.llvm.org/D86805