[ELF] Use SaveAndRestore to simplify code

[grimar]

I believe Rui was not happy when I tried to use SaveAndRestore in one of my patches.
And honestly looking at this change I think now I understand why. It is shorter, but I would not call the new code simpler.
I'll leave this up to Rui.

[ruiu]

Yeah, honestly I don't like SaveAndRestore. You can save one line per a use of SaveAndRestore, but instead everybody reading or writing the code need to learn what SaveAndRestore is. On the other hand, saving old values using local variables and assignments is really easy to read (besides that theoretically they could be overridden by operator overloading). SaveAndRestore is not a complicated class, so it's worth to learn how to use it if you have a lot of use cases of SaveAndRestore here, but three uses is too few.

ELF/ScriptParser.cpp

 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/SaveAndRestore.h"
 #include <cassert>
 #include <limits>
 #include <vector>
 
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::support::endian;
 using namespace lld;
if (Optional<std::string> Path = findFromSearchPaths(S))
       Driver->addFile(Saver.save(*Path), /*WithLOption=*/true);
     else
       setError("unable to find " + S);
   }
 }
 
 void ScriptParser::readAsNeeded() {
   expect("(");
-  bool Orig = Config->AsNeeded;
-  Config->AsNeeded = true;
+  SaveAndRestore<bool> Restore(Config->AsNeeded, true);
   while (!errorCount() && !consume(")"))
     addFile(unquote(next()));
-  Config->AsNeeded = Orig;
 }
 
 void ScriptParser::readEntry() {
   // -e <symbol> takes predecence over ENTRY(<symbol>).
   expect("(");
   StringRef Tok = next();
   if (Config->Entry.empty())
     Config->Entry = Tok;
   expect(")");
 }
 
 void ScriptParser::readExtern() {
   expect("(");
   while (!errorCount() && !consume(")"))
     Config->Undefined.push_back(next());
 }
 
 void ScriptParser::readGroup() {
-  bool Orig = InputFile::IsInGroup;
-  InputFile::IsInGroup = true;
+  SaveAndRestore<bool> Restore(InputFile::IsInGroup, true);
   readInput();
-  InputFile::IsInGroup = Orig;
-  if (!Orig)
+  if (!Restore.get())
     ++InputFile::NextGroupId;
 }
 
 void ScriptParser::readInclude() {
   StringRef Tok = unquote(next());
 
   if (!Seen.insert(Tok).second) {
     setError("there is a cycle in linker script INCLUDEs");
SymbolAssignment *ScriptParser::readSymbolAssignment(StringRef Name) {
   return make<SymbolAssignment>(Name, E, getCurrentLocation());
 }
 
 // This is an operator-precedence parser to parse a linker
 // script expression.
 Expr ScriptParser::readExpr() {
   // Our lexer is context-aware. Set the in-expression bit so that
   // they apply different tokenization rules.
-  bool Orig = InExpr;
-  InExpr = true;
+  SaveAndRestore<bool> Restore(InExpr, true);
   Expr E = readExpr1(readPrimary(), 0);
-  InExpr = Orig;
   return E;
 }
 
 Expr ScriptParser::combine(StringRef Op, Expr L, Expr R) {
   if (Op == "+")
     return [=] { return add(L(), R()); };
   if (Op == "-")
     return [=] { return sub(L(), R()); };