ObjectFileMachO: Silence signed/unsigned comparison warning

[majnemer]

Summary

File::SeekFromStart returns an off_t representing the position of the
file after seeking. This return value is always going to be one of two
values: the input or -1 in the case of failure.

ObjectFileMachO compares an expression of type off_t from the return of
File::SeekFromStart(segment.fileoff) and compares it for equality with
segment.fileoff.

The type of segment_command_64::fileoff is unsigned while off_t is
signed, comparing them emits a diagnostic under GCC.

Instead, we can just compare SeekFromSTart with -1 to see if we
successfully seeked.

[majnemer]

Closed by commit rL213822 (authored by @majnemer).

lldb/trunk/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

//                                (prot & VM_PROT_EXECUTE) ? 'x' : '-');
                         // Write the mach header and load commands out to the core file
                         size_t bytes_written = buffer.GetString().size();
                         error = core_file.Write(buffer.GetString().data(), bytes_written);
                         if (error.Success())
                         {
                             // Now write the file data for all memory segments in the process
                             for (const auto &segment : segment_load_commands)
                             {
-                                off_t offset = core_file.SeekFromStart(segment.fileoff);
-                                if (offset < 0 || segment.fileoff != static_cast<uint64_t>(offset))
+                                if (core_file.SeekFromStart(segment.fileoff) == -1)
                                 {
                                     error.SetErrorStringWithFormat("unable to seek to offset 0x%" PRIx64 " in '%s'", segment.fileoff, core_file_path.c_str());
                                     break;
                                 }
                                 
                                 printf ("Saving data for segment at 0x%" PRIx64 "\n", segment.vmaddr);
                                 addr_t bytes_left = segment.vmsize;
                                 addr_t addr = segment.vmaddr;