[Object/ELF] - Make getSymbol() return Error.

[grimar]

Summary

That is consisten with other methods around
and helps to handle error on a caller side.

include/llvm/Object/ELF.h

uint32_t getExtendedSymbolTableIndex(const Elf_Sym *Sym,
                                        const Elf_Sym *FirstSym,
                                        ArrayRef<Elf_Word> ShndxTable) const;
   const Elf_Ehdr *getHeader() const { return Header; }
   ErrorOr<const Elf_Shdr *> getSection(const Elf_Sym *Sym,
                                        const Elf_Shdr *SymTab,
                                        ArrayRef<Elf_Word> ShndxTable) const;
   ErrorOr<const Elf_Shdr *> getSection(uint32_t Index) const;
 
-  const Elf_Sym *getSymbol(const Elf_Shdr *Sec, uint32_t Index) const {
+  ErrorOr<const Elf_Sym *> getSymbol(const Elf_Shdr *Sec,
+                                     uint32_t Index) const {
     Elf_Sym_Range Symbols = symbols(Sec);
     if (Index >= Symbols.size())
-      report_fatal_error("Invalid symbol index");
+      return object_error::invalid_symbol_index;
     return &Symbols[Index];
   }
 
   ErrorOr<StringRef> getSectionName(const Elf_Shdr *Section) const;
   ErrorOr<StringRef> getSectionName(const Elf_Shdr *Section,
                                     StringRef DotShstrtab) const;
   template <typename T>
   ErrorOr<ArrayRef<T>> getSectionContentsAsArray(const Elf_Shdr *Sec) const;

include/llvm/Object/Error.h

enum class object_error {
   // Error code 0 is absent. Use std::error_code() instead.
   arch_not_found = 1,
   invalid_file_type,
   parse_failed,
   unexpected_eof,
   string_table_non_null_end,
   invalid_section_index,
   bitcode_section_not_found,
+  invalid_symbol_index,
 };
 
 inline std::error_code make_error_code(object_error e) {
   return std::error_code(static_cast<int>(e), object_category());
 }
 
 /// Base class for all errors indicating malformed binary files.
 ///

lib/Object/Error.cpp

std::string _object_error_category::message(int EV) const {
   case object_error::unexpected_eof:
     return "The end of the file was unexpectedly encountered";
   case object_error::string_table_non_null_end:
     return "String table must end with a null terminator";
   case object_error::invalid_section_index:
     return "Invalid section index";
   case object_error::bitcode_section_not_found:
     return "Bitcode section not found in object file";
+  case object_error::invalid_symbol_index:
+    return "Invalid symbol index";
   }
   llvm_unreachable("An enumerator of object_error does not have a message "
                    "defined.");
 }
 
 char BinaryError::ID = 0;
 char GenericBinaryError::ID = 0;