[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

clangd/TUScheduler.cpp

if (CanReuseAST) {
       // current file at this point?
       log("Skipping rebuild of the AST for {0}, inputs are the same.",
           FileName);
       return;
     }
     // Remove the old AST if it's still in cache.
     IdleASTs.take(this);
 
+    if (WantDiags == WantDiagnostics::No) {
+      // If no diagnostics were requested, building it in advance might be a
+      // waste if it will be removed from the cache before it is first accessed.
+      // So we choose to avoid building it in the first place, the rebuild will
+      // happen on first access instead.
+      return;
+    }
+
     // Build the AST for diagnostics.
     llvm::Optional<ParsedAST> AST =
         buildAST(FileName, std::move(Invocation), Inputs, NewPreamble, PCHs);
     // 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.
     if (WantDiags != WantDiagnostics::No && AST)
       OnUpdated(AST->getDiagnostics());