[llvm-objdump] Replace cast<> by dyn_cast<>

[Higuoxing]

Summary

cast<> can cause assertion failure, so the else routine is unreachable.

[Higuoxing]

Seems there are some legacy codes, like:

if (auto *ELF32LE = dyn_cast<ELF32LEObjectFile>(Obj))
  return getRelocationValueString(ELF32LE, Rel, Result);
if (auto *ELF64LE = dyn_cast<ELF64LEObjectFile>(Obj))
  return getRelocationValueString(ELF64LE, Rel, Result);
if (auto *ELF32BE = dyn_cast<ELF32BEObjectFile>(Obj))
  return getRelocationValueString(ELF32BE, Rel, Result);
auto *ELF64BE = cast<ELF64BEObjectFile>(Obj);
return getRelocationValueString(ELF64BE, Rel, Result);

tools/llvm-objdump/llvm-objdump.cpp

addDynamicElfSymbols(const ObjectFile *Obj,
                      std::map<SectionRef, SectionSymbolsTy> &AllSymbols) {
   assert(Obj->isELF());
   if (auto *Elf32LEObj = dyn_cast<ELF32LEObjectFile>(Obj))
     addDynamicElfSymbols(Elf32LEObj, AllSymbols);
   else if (auto *Elf64LEObj = dyn_cast<ELF64LEObjectFile>(Obj))
     addDynamicElfSymbols(Elf64LEObj, AllSymbols);
   else if (auto *Elf32BEObj = dyn_cast<ELF32BEObjectFile>(Obj))
     addDynamicElfSymbols(Elf32BEObj, AllSymbols);
-  else if (auto *Elf64BEObj = cast<ELF64BEObjectFile>(Obj))
+  else if (auto *Elf64BEObj = dyn_cast<ELF64BEObjectFile>(Obj))
     addDynamicElfSymbols(Elf64BEObj, AllSymbols);
   else
     llvm_unreachable("Unsupported binary format");
 }
 
 static void addPltEntries(const ObjectFile *Obj,
                           std::map<SectionRef, SectionSymbolsTy> &AllSymbols,
                           StringSaver &Saver) {