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

[Alexandre Ganea 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;
----------------
aganea wrote:

@MaskRay I meant this whole sequential flag could be solved by including the for... loop in the task at the call site, when serial=true, no? (in elf::scanRelocations) That way you get a valid `threadIndex` and the items are executed sequentially.
```
  bool serial = ...
  parallel::TaskGroup tg;
  auto outerFn = [&]() {
    for (ELFFileBase *f : ctx.objectFiles) {
      auto fn = [f]() {
        RelocationScanner scanner;
        for (InputSectionBase *s : f->getSections()) {
          if (s && s->kind() == SectionBase::Regular && s->isLive() &&
              (s->flags & SHF_ALLOC) &&
              !(s->type == SHT_ARM_EXIDX && config->emachine == EM_ARM))
            scanner.template scanSection<ELFT>(*s);
        }
      };
      if (serial)
        fn();
      else
        tg.spawn(fn);

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

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