[clangd] Avoid combinatorial explosion in CodeCompleteTests.

[sammccall]

Summary

This test dominates our unit test runtime, and the change speeds it up by 10x.
We lose coverage of some combinations of flags, but I'm not sure that's finding
many bugs.

3300 -> 300ms on my machine (3800 -> 800ms for the whole of CompletionTest).

[ilya-biryukov]

LGTM

clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp

EXPECT_THAT(Results.items,
               AllOf(Has("local_var"), Has("LocalClass"),
                     Contains(Kind(CompletionItemKind::Snippet))));
   // Check documentation.
   EXPECT_IFF(Opts.IncludeBriefComments, Results.items,
              Contains(IsDocumented()));
 }
 
 TEST(CompletionTest, CompletionOptions) {
-  clangd::CodeCompleteOptions Opts;
-  for (bool IncludeMacros : {true, false}) {
-    Opts.IncludeMacros = IncludeMacros;
-    for (bool IncludeGlobals : {true, false}) {
-      Opts.IncludeGlobals = IncludeGlobals;
-      for (bool IncludeBriefComments : {true, false}) {
-        Opts.IncludeBriefComments = IncludeBriefComments;
-        for (bool EnableSnippets : {true, false}) {
-          Opts.EnableSnippets = EnableSnippets;
-          for (bool IncludeCodePatterns : {true, false}) {
-            Opts.IncludeCodePatterns = IncludeCodePatterns;
-            for (bool IncludeIneligibleResults : {true, false}) {
-              Opts.IncludeIneligibleResults = IncludeIneligibleResults;
-              TestAfterDotCompletion(Opts);
-              TestGlobalScopeCompletion(Opts);
-            }
-          }
-        }
-      }
-    }
+  auto Test = [&](const clangd::CodeCompleteOptions &Opts) {
+  };
+  // We used to test every combination of options, but that got too slow (2^N).
+  auto Flags = {
+    &clangd::CodeCompleteOptions::IncludeMacros,
+    &clangd::CodeCompleteOptions::IncludeGlobals,
+    &clangd::CodeCompleteOptions::IncludeBriefComments,
+    &clangd::CodeCompleteOptions::EnableSnippets,
+    &clangd::CodeCompleteOptions::IncludeCodePatterns,
+    &clangd::CodeCompleteOptions::IncludeIneligibleResults,
+  };
+  // Test default options.
+  Test({});
+  // Test with one flag flipped.
+  for (auto &F : Flags) {
+    clangd::CodeCompleteOptions O;
+    O.*F ^= true;
+    Test(O);
   }
 }
 
 // Check code completion works when the file contents are overridden.
 TEST(CompletionTest, CheckContentsOverride) {
   MockFSProvider FS;
   IgnoreDiagnostics DiagConsumer;
   MockCompilationDatabase CDB;