[clangd] Require case-insensitive prefix match for macro completions.

[sammccall]

Summary

Macros are terribly spammy at the moment and this offers some relief.

[ioeric]

I like this.

clangd/CodeComplete.cpp

mergeResults(const std::vector<CodeCompletionResult> &SemaResults,
     for (const auto &IndexResult : IndexResults) {
       if (UsedIndexResults.count(&IndexResult))
         continue;
       addCandidate(Top, /*SemaResult=*/nullptr, &IndexResult);
     }
     return std::move(Top).items();
   }
 
+  Optional<float> fuzzyScore(const CompletionCandidate &C) {
+    // Macros can be very spammy, so we only support prefix completion.
+    if (C.SemaResult && C.SemaResult->Kind == CodeCompletionResult::RK_Macro &&
+        !C.Name.startswith_lower(Filter->pattern()))
+      return None;
+    return Filter->match(C.Name);
+  }
+
   // Scores a candidate and adds it to the TopN structure.
   void addCandidate(TopN<ScoredCandidate, ScoredCandidateGreater> &Candidates,
                     const CodeCompletionResult *SemaResult,
                     const Symbol *IndexResult) {
     CompletionCandidate C;
     C.SemaResult = SemaResult;
     C.IndexResult = IndexResult;
     C.Name = IndexResult ? IndexResult->Name : Recorder->getName(*SemaResult);
 
     SymbolQualitySignals Quality;
     SymbolRelevanceSignals Relevance;
     Relevance.Query = SymbolRelevanceSignals::CodeComplete;
-    if (auto FuzzyScore = Filter->match(C.Name))
+    if (auto FuzzyScore = fuzzyScore(C))
       Relevance.NameMatch = *FuzzyScore;
     else
       return;
     if (IndexResult) {
       Quality.merge(*IndexResult);
       Relevance.merge(*IndexResult);
     }
     if (SemaResult) {

unittests/clangd/CodeCompleteTests.cpp

int main() { ClassWithMembers().^ }
                              /*IndexSymbols=*/{}, Opts);
 
   EXPECT_TRUE(Results.isIncomplete);
   EXPECT_THAT(Results.items, ElementsAre(Named("AAA"), Named("BBB")));
 }
 
 TEST(CompletionTest, Filter) {
   std::string Body = R"cpp(
+    #define FooBarMacro
     int Abracadabra;
     int Alakazam;
     struct S {
       int FooBar;
       int FooBaz;
       int Qux;
     };
   )cpp";
   EXPECT_THAT(completions(Body + "int main() { S().Foba^ }").items,
-              AllOf(Has("FooBar"), Has("FooBaz"), Not(Has("Qux"))));
+              AllOf(Has("FooBar"), Has("FooBaz"), Not(Has("FooBarMacro")),
+                    Not(Has("Qux"))));
 
   EXPECT_THAT(completions(Body + "int main() { S().FR^ }").items,
               AllOf(Has("FooBar"), Not(Has("FooBaz")), Not(Has("Qux"))));
 
   EXPECT_THAT(completions(Body + "int main() { aaa^ }").items,
               AllOf(Has("Abracadabra"), Has("Alakazam")));
 
   EXPECT_THAT(completions(Body + "int main() { _a^ }").items,