[lldb] Print a message when background tasks take a while to complete (llvm#82799)

When terminating the debugger, we wait for all background tasks to
complete. Given that there's no way to interrupt those treads, this can
take a while. When that happens, the debugger appears to hang at exit.

The above situation is unfortunately not uncommon when background
downloading of dSYMs is enabled (`symbols.auto-download background`).
Even when calling dsymForUUID with a reasonable timeout, it can take a
while to complete.

This patch improves the user experience by printing a message from the
driver when it takes more than one (1) second to terminate the debugger.

(cherry picked from commit 2f343fc)

Author: JDevlieghere

lldb/tools/driver/Driver.cpp

@@ -32,6 +32,7 @@
 #include <bitset>
 #include <clocale>
 #include <csignal>
+#include <future>
 #include <string>
 #include <thread>
 #include <utility>
@@ -821,6 +822,18 @@ int main(int argc, char const *argv[]) {
     }
   }
 
-  SBDebugger::Terminate();
+  // When terminating the debugger we have to wait on all the background tasks
+  // to complete, which can take a while. Print a message when this takes longer
+  // than 1 second.
+  {
+    std::future<void> future =
+        std::async(std::launch::async, []() { SBDebugger::Terminate(); });
+
+    if (future.wait_for(std::chrono::seconds(1)) == std::future_status::timeout)
+      fprintf(stderr, "Waiting for background tasks to complete...\n");
+
+    future.wait();
+  }
+
   return exit_code;
 }