[lld-macho][reland] Initial support for EH Frames

This reverts commit 942f4e3a.

The additional change required to avoid the assertion errors seen
previously is:

  --- a/lld/MachO/ICF.cpp
  +++ b/lld/MachO/ICF.cpp
  @@ -443,7 +443,9 @@ void macho::foldIdenticalSections() {
                                 /*relocVA=*/0);
           isec->data = copy;
         }
  -    } else {
  +    } else if (!isEhFrameSection(isec)) {
  +      // EH frames are gathered as hashables from unwindEntry above; give a
  +      // unique ID to everything else.
         isec->icfEqClass[0] = ++icfUniqueID;
       }
     }

Differential Revision: https://reviews.llvm.org/D123435