[clangd] Do not build AST if no diagnostics were requested

[ilya-biryukov]

Summary

It can be washed out from the cache before first access anyway, so
building it can just be a waste of time.

[ioeric]

lgtm

[ilya-biryukov]

• Rebase onto head

clang-tools-extra/trunk/clangd/TUScheduler.cpp

if (!CanReuseAST) {
         // FIXME(ibiryukov): should we refresh the cache in IdleASTs for the
         // current file at this point?
         log("Skipping rebuild of the AST for {0}, inputs are the same.",
             FileName);
         return;
       }
     }
 
+    // We only need to build the AST if diagnostics were requested.
+    if (WantDiags == WantDiagnostics::No)
+      return;
+
     // Get the AST for diagnostics.
     llvm::Optional<std::unique_ptr<ParsedAST>> AST = IdleASTs.take(this);
     if (!AST) {
       llvm::Optional<ParsedAST> NewAST =
           buildAST(FileName, std::move(Invocation), Inputs, NewPreamble, PCHs);
       AST = NewAST ? llvm::make_unique<ParsedAST>(std::move(*NewAST)) : nullptr;
     }
-
     // We want to report the diagnostics even if this update was cancelled.
     // It seems more useful than making the clients wait indefinitely if they
     // spam us with updates.
     // Note *AST can be still be null if buildAST fails.
-    if (WantDiags != WantDiagnostics::No && *AST) {
+    if (*AST) {
       OnUpdated((*AST)->getDiagnostics());
       DiagsWereReported = true;
     }
     // Stash the AST in the cache for further use.
     IdleASTs.put(this, std::move(*AST));
   };
 
   startTask("Update", Bind(Task, std::move(OnUpdated)), WantDiags);