[llvm] Teach LLVM about filesets

Teach LLVM about filesets. Filesets were added in macOS 11 (Big Sur) to
combine multiple Mach-O files. They introduce a new load command
(LC_FILESET_ENTRY) consisting of a fileset_entry_command.

  struct fileset_entry_command {
      uint32_t     cmd;        /* LC_FILESET_ENTRY */
      uint32_t     cmdsize;    /* includes entry_id string */
      uint64_t     vmaddr;     /* memory address of the entry */
      uint64_t     fileoff;    /* file offset of the entry */
      union lc_str entry_id;   /* contained entry id */
      uint32_t     reserved;   /* reserved */
  };

This patch teaches LLVM about the new load command and the corresponding
data.

Differential revision: https://reviews.llvm.org/D132432