[BOLT] Avoid creating a temporary instance of std::string (NFC) (#140987)

lookupTarget takes StringRef and internally creates an instance of
std::string with the StringRef as part of constructing Triple, so we
don't need to create a temporary instance of std::string on our own.

Author: kazutakahirata

bolt/lib/Core/BinaryContext.cpp

@@ -201,7 +201,7 @@ Expected<std::unique_ptr<BinaryContext>> BinaryContext::createBinaryContext(
 
   std::string Error;
   const Target *TheTarget =
-      TargetRegistry::lookupTarget(std::string(ArchName), TheTriple, Error);
+      TargetRegistry::lookupTarget(ArchName, TheTriple, Error);
   if (!TheTarget)
     return createStringError(make_error_code(std::errc::not_supported),
                              Twine("BOLT-ERROR: ", Error));