[llvm-objcopy] Always check for indirect symbol references before stripping

[int3]

Summary

Previously, we only checked for those references when doing
--strip-all, but not when doing --discard-all. I believe that this
didn't cause issues with ld64-generated binaries as they did not create
indirect symbol references to local symbols (which --discard-all
removes). However LLD does emit these, exposing the issue.

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

 
 ## Verify that llvm-strip modifies the symbol table the same way.
 # RUN: cp %t %t4
 # RUN: llvm-strip --discard-all %t4
 # RUN: cmp %t2 %t4
 # RUN: llvm-strip -x %t
 # RUN: cmp %t2 %t
 
+## FIXME: We should also check that we don't discard symbols that are referenced
+## by the indirect symbol table. However, yaml2obj currently lacks support for
+## the indirect symbol table.
+
 # CHECK:       Symbols [
 # CHECK-NEXT:    Symbol {
 # CHECK-NEXT:      Name: _b (1)
 # CHECK-NEXT:      Extern
 # CHECK-NEXT:      Type: Undef (0x0)
 # CHECK-NEXT:      Section:  (0x0)
 # CHECK-NEXT:      RefType: UndefinedNonLazy (0x0)
 # CHECK-NEXT:      Flags [ (0x220)

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

for (StringRef Flag : Config.DumpSection) {
     if (Error E = dumpSectionToFile(SectionName, FileName, Obj))
       return E;
   }
 
   if (Error E = removeSections(Config, Obj))
     return E;
 
   // Mark symbols to determine which symbols are still needed.
-  if (Config.StripAll)
-    markSymbols(Config, Obj);
-
   updateAndRemoveSymbols(Config, Obj);
 
   if (Config.StripAll)
     for (LoadCommand &LC : Obj.LoadCommands)
       for (std::unique_ptr<Section> &Sec : LC.Sections)
         Sec->Relocations.clear();
 
   for (const auto &Flag : Config.AddSection) {