[clangd] Make stable_partition on include candidates less slow. NFC

[ioeric]

Summary

stable_partition on objects is slow (due to copies). Do it on pointers
instead.

[sammccall]

clangd/CodeComplete.cpp

326

this seems a bit overly complicated. It does seem like a worry that this code is hot enough to optimize, especially compared to *generating* the list.

But I think we can do something simpler...

410

if this is really hot, you might want to reserve(C.RankedIncludeHeaders.size())

419–421

What about
(Include.Insertion ? InsertableCandidates : IncludeCandidates).push_back(std::move(Include))

where InsertableCandidates is a vector declared above, and then just move it onto the end of the list after the loop?

[ioeric]

• simplify the code.

[sammccall]

The implementation looks fine, but I believe we concluded that seeing huge lists here was basically a bug.
So you may not want to land it, if you prefer to keep things simple. Up to you.

clangd/CodeComplete.cpp

if (L.second.Total != R.second.Total)
       return L.second.Total > R.second.Total;
     return L.first.front().Name <
            R.first.front().Name; // Earlier name is better.
   }
 };
 
 // Assembles a code completion out of a bundle of >=1 completion candidates.
 // Many of the expensive strings are only computed at this point, once we know
 // the candidate bundle is going to be returned.
 //
 // Many fields are the same for all candidates in a bundle (e.g. name), and are
 // computed from the first candidate, in the constructor.
 // Others vary per candidate, so add() must be called for remaining candidates.
 struct CodeCompletionBuilder {
   CodeCompletionBuilder(ASTContext &ASTCtx, const CompletionCandidate &C,
                         CodeCompletionString *SemaCCS,
                         llvm::ArrayRef<std::string> QueryScopes,
auto Inserted =
       auto ResolvedInserted = toHeaderFile(Header, FileName);
       if (!ResolvedInserted)
         return ResolvedInserted.takeError();
       return std::make_pair(
           Includes.calculateIncludePath(*ResolvedDeclaring, *ResolvedInserted),
           Includes.shouldInsertInclude(*ResolvedDeclaring, *ResolvedInserted));
     };
     bool ShouldInsert = C.headerToInsertIfAllowed().hasValue();
-    // Calculate include paths and edits for all possible headers.
+    // Put includes with insertions after those without.
+    llvm::SmallVector<CodeCompletion::IncludeCandidate, 1> InsertableCandidates;
+    Completion.Includes.reserve(C.RankedIncludeHeaders.size());
+    InsertableCandidates.reserve(C.RankedIncludeHeaders.size());
     for (const auto &Inc : C.RankedIncludeHeaders) {
       if (auto ToInclude = Inserted(Inc)) {
         CodeCompletion::IncludeCandidate Include;
         Include.Header = ToInclude->first;
         if (ToInclude->second && ShouldInsert)
           Include.Insertion = Includes.insert(ToInclude->first);
-        Completion.Includes.push_back(std::move(Include));
+        (Include.Insertion ? InsertableCandidates : Completion.Includes)
+            .push_back(std::move(Include));
+
       } else
         log("Failed to generate include insertion edits for adding header "
             "(FileURI='{0}', IncludeHeader='{1}') into {2}",
             C.IndexResult->CanonicalDeclaration.FileURI, Inc, FileName);
     }
-    // Prefer includes that do not need edits (i.e. already exist).
-    std::stable_partition(Completion.Includes.begin(),
-                          Completion.Includes.end(),
-                          [](const CodeCompletion::IncludeCandidate &I) {
-                            return !I.Insertion.hasValue();
-                          });
+    if (!InsertableCandidates.empty())
+      std::move(InsertableCandidates.begin(), InsertableCandidates.end(),
+                std::back_inserter(Completion.Includes));
   }
 
   void add(const CompletionCandidate &C, CodeCompletionString *SemaCCS) {
     assert(bool(C.SemaResult) == bool(SemaCCS));
     Bundled.emplace_back();
     BundledEntry &S = Bundled.back();
     if (C.SemaResult) {
       getSignature(*SemaCCS, &S.Signature, &S.SnippetSuffix,