[MachO] Support exports trie in both LC_DYLD_INFO and LC_DYLD_EXPORTS_TRIE

The exports trie used to be pointed by the information in LC_DYLD_INFO,
but when chained fixups are present, the exports trie is pointed by
LC_DYLD_EXPORTS_TRIE instead.

Modify the Object library to give access to the information pointed by
each of the load commands, and to fallback from one into the other when
the exports are requested.

Modify ObjectYAML to support dumping the export trie when pointed by
LC_DYLD_EXPORTS_TRIE and to parse the existence of a export trie also
when the load command is present.

This is a split of D134250 with improvements on top.

Reviewed By: alexander-shaposhnikov

Differential Revision: https://reviews.llvm.org/D134571