[llvm-objcopy][MachO] Fix symbol order in the symbol table

[MaskRay]

Summary

Only consider isUndefinedSymbol() when the symbol is not local. This
fixes an assert failure when copying the symbol table, if a n_type=0x20
symbol is followed by a n_type=0x64 symbol.

[seiya]

LGTM too. Thanks for the fix!

llvm/test/tools/llvm-objcopy/MachO/Inputs/strip-all.yaml

- n_strx:          1
       n_sect:          1
       n_desc:          0
       n_value:         36
     - n_strx:          170
       n_type:          0x26
       n_sect:          3
       n_desc:          0
       n_value:         4294971392
+    ## A local undefined SymDebugTable entry followed by
     - n_strx:          175
       n_type:          0x20
       n_sect:          0
       n_desc:          0
       n_value:         0
+    ## a defined SymDebugTable entry.
     - n_strx:          1
       n_type:          0x64
       n_sect:          1
       n_desc:          0
       n_value:         0
     - n_strx:          180
       n_type:          0x0E
       n_sect:          3

llvm/test/tools/llvm-objcopy/MachO/strip-all.test

 ## Show that llvm-objcopy/llvm-strip removes all symbols and debug sections.
 
 # RUN: yaml2obj %p/Inputs/strip-all.yaml > %t.exec
 # RUN: yaml2obj %p/Inputs/strip-all-with-dwarf.yaml > %t.dwarf
 
+## Check that the symbol list satisfies the order: local / defined external /
+## undefined external, otherwise llvm-objcopy will fail.
+# RUN: llvm-objcopy %t.exec /dev/null
+
 # RUN: llvm-objcopy --strip-all %t.exec %t.exec.stripped
 # RUN: llvm-readobj --sections --relocations --symbols %t.exec.stripped \
 # RUN:   | FileCheck --check-prefixes=COMMON,EXEC %s
 
 # RUN: llvm-objcopy --strip-all %t.dwarf %t.dwarf.stripped
 # RUN: llvm-readobj --sections --relocations --symbols %t.dwarf.stripped \
 # RUN:   | FileCheck --check-prefixes=COMMON,DWARF %s
 

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

 // Updates the index and the number of local/external/undefined symbols.
 void MachOLayoutBuilder::updateDySymTab(MachO::macho_load_command &MLC) {
   assert(MLC.load_command_data.cmd == MachO::LC_DYSYMTAB);
   // Make sure that nlist entries in the symbol table are sorted by the those
   // types. The order is: local < defined external < undefined external.
   assert(std::is_sorted(O.SymTable.Symbols.begin(), O.SymTable.Symbols.end(),
                         [](const std::unique_ptr<SymbolEntry> &A,
                            const std::unique_ptr<SymbolEntry> &B) {
-                          return (A->isLocalSymbol() && !B->isLocalSymbol()) ||
-                                 (!A->isUndefinedSymbol() &&
-                                  B->isUndefinedSymbol());
+                          bool AL = A->isLocalSymbol(), BL = B->isLocalSymbol();
+                          if (AL != BL)
+                            return AL;
+                          return !AL && !A->isUndefinedSymbol() &&
+                                         B->isUndefinedSymbol();
                         }) &&
          "Symbols are not sorted by their types.");
 
   uint32_t NumLocalSymbols = 0;
   auto Iter = O.SymTable.Symbols.begin();
   auto End = O.SymTable.Symbols.end();
   for (; Iter != End; ++Iter) {
     if ((*Iter)->isExternalSymbol())