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

[Fangrui Song 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;
----------------
MaskRay wrote:

I think this suggestion is what I previously did (see the deleted code on https://reviews.llvm.org/D148728)
```cpp
    if (serial)
      fn();
    else
      tg.execute(fn);
```

It seems convenient to have a Parallel helper function to simplify this pattern. I do agree that the load-bearing `spawn` should not have extra overhead (`bool Sequential = false`) but that is not the focus of this patch.

---

Perhaps the next patch can add:

```
spawn(std::function f)
execute(std::function f, bool serial)
```

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