[llvm-objcopy] Fix misaligned access to load command data.

[drodriguez]

Summary

It seems that llvm-objcopy stores data temporarily misaligned with the
requirements of the underlaying struct from libBinaryFormat, and UBSan
generates a runtime error.

Instead of trying to reinterpret the memory as the struct itself, simply
access the char * pointer that we are interested in, and that do not
have alignment restrictions.

This problem was pointed out in a comment of D111164.

llvm/tools/llvm-objcopy/MachO/MachOReader.cpp

Error MachOReader::readLoadCommands(Object &O) const {
   static constexpr char TextSegmentName[] = "__TEXT";
   for (auto LoadCmd : MachOObj.load_commands()) {
     LoadCommand LC;
     switch (LoadCmd.C.cmd) {
     case MachO::LC_CODE_SIGNATURE:
       O.CodeSignatureCommandIndex = O.LoadCommands.size();
       break;
     case MachO::LC_SEGMENT:
-      if (StringRef(
-              reinterpret_cast<MachO::segment_command const *>(LoadCmd.Ptr)
-                  ->segname) == TextSegmentName)
+      // LoadCmd.Ptr might not be aligned temporarily as
+      // MachO::segment_command requires, but the segname char pointer do not
+      // have alignment restrictions.
+      if (StringRef(reinterpret_cast<const char *>(
+              LoadCmd.Ptr + offsetof(MachO::segment_command, segname))) ==
+          TextSegmentName)
         O.TextSegmentCommandIndex = O.LoadCommands.size();
 
       if (Expected<std::vector<std::unique_ptr<Section>>> Sections =
               extractSections<MachO::section, MachO::segment_command>(
                   LoadCmd, MachOObj, NextSectionIndex))
         LC.Sections = std::move(*Sections);
       else
         return Sections.takeError();
       break;
     case MachO::LC_SEGMENT_64:
-      if (StringRef(
-              reinterpret_cast<MachO::segment_command_64 const *>(LoadCmd.Ptr)
-                  ->segname) == TextSegmentName)
+      // LoadCmd.Ptr might not be aligned temporarily as
+      // MachO::segment_command_64 requires, but the segname char pointer do
+      // not have alignment restrictions.
+      if (StringRef(reinterpret_cast<const char *>(
+              LoadCmd.Ptr + offsetof(MachO::segment_command_64, segname))) ==
+          TextSegmentName)
         O.TextSegmentCommandIndex = O.LoadCommands.size();
 
       if (Expected<std::vector<std::unique_ptr<Section>>> Sections =
               extractSections<MachO::section_64, MachO::segment_command_64>(
                   LoadCmd, MachOObj, NextSectionIndex))
         LC.Sections = std::move(*Sections);
       else
         return Sections.takeError();