[llvm-objcopy] Remove references for empty section groups

Otherwise, llvm-objcopy fails with use-after-free when built under
sanitizers. Simple repro can be running the test ELF/remove-section-in-group.test
under asan. This is due to symbol table references to empty section groups.

Author: pranavk

llvm/lib/ObjCopy/ELF/ELFObject.cpp

@@ -2203,6 +2203,11 @@ Error Object::removeSections(
           if (auto ToRelSec = RelSec->getSection())
             return !ToRemove(*ToRelSec);
         }
+        // Remove empty group sections.
+        if (Sec->Type == ELF::SHT_GROUP) {
+          auto GroupSec = cast<GroupSection>(Sec.get());
+          return !llvm::all_of(GroupSec->members(), ToRemove);
+        }
         return true;
       });
   if (SymbolTable != nullptr && ToRemove(*SymbolTable))

llvm/lib/ObjCopy/ELF/ELFObject.h

@@ -941,6 +941,9 @@ class GroupSection : public SectionBase {
   SmallVector<SectionBase *, 3> GroupMembers;
 
 public:
+  template <class T>
+  using ConstRange = iterator_range<
+      pointee_iterator<typename llvm::SmallVector<T *, 3>::const_iterator>>;
   // TODO: Contents is present in several classes of the hierarchy.
   // This needs to be refactored to avoid duplication.
   ArrayRef<uint8_t> Contents;
@@ -964,6 +967,10 @@ class GroupSection : public SectionBase {
       const DenseMap<SectionBase *, SectionBase *> &FromTo) override;
   void onRemove() override;
 
+  ConstRange<SectionBase> members() const {
+    return make_pointee_range(GroupMembers);
+  }
+
   static bool classof(const SectionBase *S) {
     return S->OriginalType == ELF::SHT_GROUP;
   }