Use ArrayRef::copy, instead of copying data manually

[ioeric]

Nice!

[lebedev.ri]

Would it please be possible to write actual commit / DR titles,
i.e. include appropriate [tag]s into them,
and ideally proper commit messages, too?
It really clutters -commit list listing otherwise.

clangd/index/Ref.cpp

RefSlab RefSlab::Builder::build() && {
   size_t NumRefs = 0;
   for (auto &Sym : Refs) {
     auto &SymRefs = Sym.second;
     llvm::sort(SymRefs);
     // FIXME: do we really need to dedup?
     SymRefs.erase(std::unique(SymRefs.begin(), SymRefs.end()), SymRefs.end());
 
     NumRefs += SymRefs.size();
-    auto *Array = Arena.Allocate<Ref>(SymRefs.size());
-    std::uninitialized_copy(SymRefs.begin(), SymRefs.end(), Array);
-    Result.emplace_back(Sym.first, llvm::ArrayRef<Ref>(Array, SymRefs.size()));
+    Result.emplace_back(Sym.first, llvm::ArrayRef<Ref>(SymRefs).copy(Arena));
   }
   return RefSlab(std::move(Result), std::move(Arena), NumRefs);
 }
 
 } // namespace clangd
 } // namespace clang