[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:

This patch is an almost revert of reviews.llvm.org/D148728 . 

Since the use case is so special, I'm fine with the style suggested by @aganea (also the initial style I used for parallel relocation scanning):

```cpp
      if (serial)
        fn();
      else
        tg.spawn(fn);

    }
  };
  if (serial)
    tg.spawn(outerFn);
  else
    outerFn();
```

Perhaps Parallel.cpp could add `setThreadIndex` and lld/ELF/Relocations.cpp could call `setThreadIndex(0)` to make the pattern more explicit. It's up to lld to ensure correctness.

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