[BOLT][NFC] Use refs for loop variables to avoid copies

[Amir]

Summary

Addresses warnings when built with Apple Clang.

bolt/lib/Core/BinaryFunction.cpp

void BinaryFunction::print(raw_ostream &OS, std::string Annotation,
 
   StringRef SectionName =
       OriginSection ? OriginSection->getName() : "<no origin section>";
   OS << "Binary Function \"" << *this << "\" " << Annotation << " {";
   std::vector<StringRef> AllNames = getNames();
   if (AllNames.size() > 1) {
     OS << "\n  All names   : ";
     const char *Sep = "";
-    for (const StringRef Name : AllNames) {
+    for (const StringRef &Name : AllNames) {
       OS << Sep << Name;
       Sep = "\n                ";
     }
   }
   OS << "\n  Number      : "   << FunctionNumber
      << "\n  State       : "   << CurrentState
      << "\n  Address     : 0x" << Twine::utohexstr(Address)
      << "\n  Size        : 0x" << Twine::utohexstr(Size)

bolt/lib/Profile/DataReader.cpp

if ((LTOSuffixPos = Name.find(".llvm.")) != StringRef::npos)
     return Name.substr(0, LTOSuffixPos + 6);
   return NoneType();
 }
 
 namespace {
 
 /// Return true if the function name can change across compilations.
 bool hasVolatileName(const BinaryFunction &BF) {
-  for (const StringRef Name : BF.getNames())
+  for (const StringRef &Name : BF.getNames())
     if (getLTOCommonName(Name))
       return true;
 
   return false;
 }
 
 /// Return standard escaped name of the function possibly renamed by BOLT.
 std::string normalizeName(StringRef NameRef) {

bolt/lib/Rewrite/BoltDiff.cpp

class RewriteInstanceDiff {
   /// later when matching functions in binary 2 to corresponding functions
   /// in binary 1
   void buildLookupMaps() {
     for (const auto &BFI : RI1.BC->getBinaryFunctions()) {
       StringRef LTOName;
       const BinaryFunction &Function = BFI.second;
       const double Score = getNormalizedScore(Function, RI1);
       LargestBin1.insert(std::make_pair<>(Score, &Function));
-      for (const StringRef Name : Function.getNames()) {
+      for (const StringRef &Name : Function.getNames()) {
         if (Optional<StringRef> OptionalLTOName = getLTOCommonName(Name))
           LTOName = *OptionalLTOName;
         NameLookup[Name] = &Function;
       }
       if (opts::MatchByHash && Function.hasCFG())
         HashLookup[Function.computeHash(/*UseDFS=*/true)] = &Function;
       if (opts::IgnoreLTOSuffix && !LTOName.empty()) {
         if (!LTONameLookup1.count(LTOName))
           LTONameLookup1[LTOName] = &Function;
         LTOMap1[&Function] = LTONameLookup1[LTOName];
       }
     }
 
     // Compute LTONameLookup2 and LargestBin2
     for (const auto &BFI : RI2.BC->getBinaryFunctions()) {
       StringRef LTOName;
       const BinaryFunction &Function = BFI.second;
       const double Score = getNormalizedScore(Function, RI2);
       LargestBin2.insert(std::make_pair<>(Score, &Function));
-      for (const StringRef Name : Function.getNames()) {
+      for (const StringRef &Name : Function.getNames()) {
         if (Optional<StringRef> OptionalLTOName = getLTOCommonName(Name))
           LTOName = *OptionalLTOName;
       }
       if (opts::IgnoreLTOSuffix && !LTOName.empty()) {
         if (!LTONameLookup2.count(LTOName))
           LTONameLookup2[LTOName] = &Function;
         LTOMap2[&Function] = LTONameLookup2[LTOName];
       }
     }
   }
 
   /// Match functions in binary 2 with functions in binary 1
   void matchFunctions() {
     outs() << "BOLT-DIFF: Mapping functions in Binary2 to Binary1\n";
     uint64_t BothHaveProfile = 0ull;
     std::set<const BinaryFunction *> Bin1ProfiledMapped;
 
     for (const auto &BFI2 : RI2.BC->getBinaryFunctions()) {
       const BinaryFunction &Function2 = BFI2.second;
       StringRef LTOName;
       bool Match = false;
-      for (const StringRef Name : Function2.getNames()) {
+      for (const StringRef &Name : Function2.getNames()) {
         auto Iter = NameLookup.find(Name);
         if (Optional<StringRef> OptionalLTOName = getLTOCommonName(Name))
           LTOName = *OptionalLTOName;
         if (Iter == NameLookup.end())
           continue;
         FuncMap.insert(std::make_pair<>(&Function2, Iter->second));
         Bin1MappedFuncs.insert(Iter->second);
         Bin2MappedFuncs.insert(&Function2);