update ManualDWARFIndex::Index to use std::once (#165896)

Small change to use (what I think is) a better practice -- we were using
the `m_indexed` bool member to make sure we called `Index()` once, but
we should just use `std::once`! This change shouldn't affect
functionality.

This change may also make concurrent access to `Index()` thread-safe,
though the ManualDWARFIndex API isn't completely thread-safe due to
`Decode()`. I'm not sure if ManualDWARFIndex was ever intended to be
thread-safe.

Test Plan:

`ninja check-lldb`

Tested basic debugging workflow of a couple of large projects I had
built. Basically:
```
(lldb) target create <project>
(lldb) b main
(lldb) r
(lldb) step
... 
```

I A/B tested the performance of launching several modules with parallel
module loading and didn't observe any performance regressions.

---------

Co-authored-by: Tom Yang <toyang@fb.com>

lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.cpp

diff --git a/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.cpp b/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.cpp
index d90108f..36dee14 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.cpp
@@ -22,7 +22,6 @@
 #include "lldb/Utility/Stream.h"
 #include "lldb/Utility/Timer.h"
 #include "lldb/lldb-private-enumerations.h"
-#include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/ThreadPool.h"
 #include <atomic>
 #include <optional>
@@ -33,10 +32,10 @@
 using namespace llvm::dwarf;
 
 void ManualDWARFIndex::Index() {
-  if (m_indexed)
-    return;
-  m_indexed = true;
+  std::call_once(m_indexed_flag, [this]() { IndexImpl(); });
+}
 
+void ManualDWARFIndex::IndexImpl() {
   ElapsedTime elapsed(m_index_time);
   LLDB_SCOPED_TIMERF("%p", static_cast<void *>(m_dwarf));
   if (LoadFromCache()) {

lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.h

diff --git a/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.h b/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.h
index 0b5b2f3..41e0e62 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/ManualDWARFIndex.h
@@ -66,8 +66,14 @@
   void Dump(Stream &s) override;
 
 private:
+  /// Reads the DWARF debug info to build the index once.
+  ///
+  /// Should be called before attempting to retrieve symbols.
   void Index();
 
+  /// Call `ManualDWARFIndex::Index()` instead.
+  void IndexImpl();
+
   /// Decode a serialized version of this object from data.
   ///
   /// \param data
@@ -170,7 +176,7 @@
   llvm::DenseSet<uint64_t> m_type_sigs_to_avoid;
 
   IndexSet<NameToDIE> m_set;
-  bool m_indexed = false;
+  std::once_flag m_indexed_flag;
 };
 } // namespace dwarf
 } // namespace lldb_private::plugin