[BOLT] Fix build with GCC 7.3.0

[yota9]

Summary

The gcc 7.3.0 version raises "could not covert" error without std::move
used explicitly.

bolt/lib/Core/BinaryContext.cpp

BinaryContext::createBinaryContext(const ObjectFile *File, bool IsPIC,
 
   BC->MAB = std::unique_ptr<MCAsmBackend>(
       BC->TheTarget->createMCAsmBackend(*BC->STI, *BC->MRI, MCTargetOptions()));
 
   BC->setFilename(File->getFileName());
 
   BC->HasFixedLoadAddress = !IsPIC;
 
-  return BC;
+  return std::move(BC);
 }
 
 bool BinaryContext::forceSymbolRelocations(StringRef SymbolName) const {
   if (opts::HotText &&
       (SymbolName == "__hot_start" || SymbolName == "__hot_end"))
     return true;
 
   if (opts::HotData &&

bolt/lib/Rewrite/MachORewriteInstance.cpp

 Expected<std::unique_ptr<MachORewriteInstance>>
 MachORewriteInstance::createMachORewriteInstance(
     object::MachOObjectFile *InputFile, StringRef ToolPath) {
   Error Err = Error::success();
   auto MachORI =
       std::make_unique<MachORewriteInstance>(InputFile, ToolPath, Err);
   if (Err)
     return std::move(Err);
-  return MachORI;
+  return std::move(MachORI);
 }
 
 MachORewriteInstance::MachORewriteInstance(object::MachOObjectFile *InputFile,
                                            StringRef ToolPath, Error &Err)
     : InputFile(InputFile), ToolPath(ToolPath) {
   ErrorAsOutParameter EAO(&Err);
   auto BCOrErr = BinaryContext::createBinaryContext(
       InputFile, /* IsPIC */ true, DWARFContext::create(*InputFile));

bolt/lib/Rewrite/RewriteInstance.cpp

 Expected<std::unique_ptr<RewriteInstance>>
 RewriteInstance::createRewriteInstance(ELFObjectFileBase *File, const int Argc,
                                        const char *const *Argv,
                                        StringRef ToolPath) {
   Error Err = Error::success();
   auto RI = std::make_unique<RewriteInstance>(File, Argc, Argv, ToolPath, Err);
   if (Err)
     return std::move(Err);
-  return RI;
+  return std::move(RI);
 }
 
 RewriteInstance::RewriteInstance(ELFObjectFileBase *File, const int Argc,
                                  const char *const *Argv, StringRef ToolPath,
                                  Error &Err)
     : InputFile(File), Argc(Argc), Argv(Argv), ToolPath(ToolPath),
       SHStrTab(StringTableBuilder::ELF) {
   ErrorAsOutParameter EAO(&Err);