[clangd] Respect WantDiags when emitting diags from possibly stale preambles

clang-tools-extra/clangd/TUScheduler.cpp

auto Task = [=]() mutable {
     // can be built concurrently.
     PreamblePeer.update(std::make_unique<CompilerInvocation>(*Invocation),
                         Inputs, CompilerInvocationDiags, WantDiags);
 
     // Emit diagnostics from (possibly) stale preamble while waiting for a
     // rebuild. Newly built preamble cannot emit diagnostics before this call
     // finishes (ast callbacks are called from astpeer thread), hence we
     // gurantee eventual consistency.
-    if (LatestPreamble && Config::current().Diagnostics.AllowStalePreamble)
+    if (LatestPreamble && WantDiags != WantDiagnostics::No &&
+        Config::current().Diagnostics.AllowStalePreamble)
       generateDiagnostics(std::move(Invocation), std::move(Inputs),
                           std::move(CompilerInvocationDiags));
 
     std::unique_lock<std::mutex> Lock(Mutex);
     PreambleCV.wait(Lock, [this] {
       // Block until we reiceve a preamble request, unless a preamble already
       // exists, as patching an empty preamble would imply rebuilding it from
       // scratch.

clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp

TEST_F(TUSchedulerTests, PublishWithStalePreamble) {
   PI.Contents = "#define FOO\n" + PI.Version;
   ASSERT_THAT(BlockForDiags(PI), testing::Pair("1", "3"));
 
   UnblockPreamble.notify();
   S.blockUntilIdle(timeoutSeconds(5));
 
   // Make sure that we have eventual consistency.
   EXPECT_THAT(Collector.diagVersions().back(), Pair(PI.Version, PI.Version));
+
+  // Check that WantDiagnostics::No doesn't emit any diags.
+  PI.Version = "4";
+  PI.Contents = "#define FOO\n" + PI.Version;
+  S.update(File, PI, WantDiagnostics::No);
+  S.blockUntilIdle(timeoutSeconds(5));
+  EXPECT_THAT(Collector.diagVersions().back(), Pair("3", "3"));
 }
 
 // If a header file is missing from the CDB (or inferred using heuristics), and
 // it's included by another open file, then we parse it using that files flags.
 TEST_F(TUSchedulerTests, IncluderCache) {
   static std::string Main = testPath("main.cpp"), Main2 = testPath("main2.cpp"),
                      Main3 = testPath("main3.cpp"),
                      NoCmd = testPath("no_cmd.h"),