[clangd] Fix broken assertion

[kadircet]

Summary

This assertion was bad. It will show up once we start running preamble
thread async. Think about the following case:

• Update 1 builds a preamble, and an AST. Caches the AST.
• Update 2 Invalidates the cache, preamble hasn't changed.
• Update 3 Invalidates the cache, preamble hasn't changed
• Read builds AST using preamble v1, and caches it. preamble for v2 gets build, cache isn't invalidated since preamble is same. generateDiags tries to reuse cached AST but latest version is 3 not 2, so assertion fails.

[kadircet]

• Delete the assertion too

clang-tools-extra/clangd/TUScheduler.cpp

void ASTWorker::generateDiagnostics(
   Status.update([&](TUStatus &Status) {
     Status.ASTActivity.K = ASTAction::Building;
     Status.ASTActivity.Name = std::move(TaskName);
   });
   // We might be able to reuse the last we've built for a read request.
   // FIXME: It might be better to not reuse this AST. That way queued AST builds
   // won't be required for diags.
   llvm::Optional<std::unique_ptr<ParsedAST>> AST = IdleASTs.take(this);
-  if (!AST) {
+  if (!AST || !InputsAreLatest) {
     auto RebuildStartTime = DebouncePolicy::clock::now();
     llvm::Optional<ParsedAST> NewAST = buildAST(
         FileName, std::move(Invocation), CIDiags, Inputs, LatestPreamble);
     auto RebuildDuration = DebouncePolicy::clock::now() - RebuildStartTime;
     // Try to record the AST-build time, to inform future update debouncing.
     // This is best-effort only: if the lock is held, don't bother.
     std::unique_lock<std::mutex> Lock(Mutex, std::try_to_lock);
     if (Lock.owns_lock()) {
       // Do not let RebuildTimes grow beyond its small-size (i.e.
       // capacity).
       if (RebuildTimes.size() == RebuildTimes.capacity())
         RebuildTimes.erase(RebuildTimes.begin());
       RebuildTimes.push_back(RebuildDuration);
       Lock.unlock();
     }
     Status.update([&](TUStatus &Status) {
       Status.Details.ReuseAST = false;
       Status.Details.BuildFailed = !NewAST.hasValue();
     });
     AST = NewAST ? std::make_unique<ParsedAST>(std::move(*NewAST)) : nullptr;
   } else {
-    assert(InputsAreLatest && !RanASTCallback &&
-           "forgot to invalidate cached ast?");
     log("Skipping rebuild of the AST for {0}, inputs are the same.", FileName);
     Status.update([](TUStatus &Status) {
       Status.Details.ReuseAST = true;
       Status.Details.BuildFailed = false;
     });
   }
 
   // Publish diagnostics.