[libObject][NFC] Use sys::path::convert_to_slash.

[rupprecht]

Summary

As suggested in rL353995

[compnerd]

Thanks!

llvm/lib/Object/ArchiveWriter.cpp

std::string computeArchiveRelativePath(StringRef From, StringRef To) {
 
   SmallString<128> Relative;
   for (auto FromE = sys::path::end(DirFrom); FromI != FromE; ++FromI)
     sys::path::append(Relative, "..");
 
   for (auto ToE = sys::path::end(To); ToI != ToE; ++ToI)
     sys::path::append(Relative, *ToI);
 
-#ifdef _WIN32
   // Replace backslashes with slashes so that the path is portable between *nix
   // and Windows.
-  std::replace(Relative.begin(), Relative.end(), '\\', '/');
-#endif
-
-  return Relative.str();
+  return sys::path::convert_to_slash(Relative);
 }
 
 Error writeArchive(StringRef ArcName, ArrayRef<NewArchiveMember> NewMembers,
                    bool WriteSymtab, object::Archive::Kind Kind,
                    bool Deterministic, bool Thin,
                    std::unique_ptr<MemoryBuffer> OldArchiveBuf) {
   assert((!Thin || !isBSDLike(Kind)) && "Only the gnu format has a thin mode");