[ELF] - Do not crash when proccessing absolute symbols during GC.

[grimar]

Summary

This is PR33239, previously we segfault on given testcase.

[smeenai]

Thanks!

ELF/MarkLive.cpp

auto Enqueue = [&](ResolvedReloc R) {
     R.Sec->Live = true;
     // Add input section to the queue.
     if (InputSection *S = dyn_cast<InputSection>(R.Sec))
       Q.push_back(S);
   };
 
   auto MarkSymbol = [&](const SymbolBody *Sym) {
     if (auto *D = dyn_cast_or_null<DefinedRegular>(Sym))
-      Enqueue({cast<InputSectionBase>(D->Section), D->Value});
+      if (auto *IS = dyn_cast_or_null<InputSectionBase>(D->Section))
+        Enqueue({IS, D->Value});
   };
 
   // Add GC root symbols.
   MarkSymbol(Symtab<ELFT>::X->find(Config->Entry));
   MarkSymbol(Symtab<ELFT>::X->find(Config->Init));
   MarkSymbol(Symtab<ELFT>::X->find(Config->Fini));
   for (StringRef S : Config->Undefined)
     MarkSymbol(Symtab<ELFT>::X->find(S));

test/ELF/gc-absolute.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t2 -shared --gc-sections
+
+.global foo
+foo = 0x123