[llvm] [Parallel] Ensure getThreadIndex()==0 for sequential tasks (PR #109084)

[Andrew Ng via llvm-commits]

================
@@ -217,13 +194,18 @@ TaskGroup::~TaskGroup() {
 void TaskGroup::spawn(std::function<void()> F, bool Sequential) {
 #if LLVM_ENABLE_THREADS
   if (Parallel) {
+    if (Sequential) {
+      // Act as worker thread 0.
+      threadIndex = 0;
----------------
nga888 wrote:

IIUC, if a user was to mix sequential and parallel tasks within the same `TaskGroup` then this could potentially lead to unintentional concurrent access to thread index `0` resources.

https://github.com/llvm/llvm-project/pull/109084