Clean up the GSym error aggregation code, and pass the aggregator by reference (#89688)

There was a problem with `llvm-gsymutil`s error aggregation code not
properly collecting aggregate errors. The was that the output aggregator
collecting errors from other threads wasn't being passed by reference,
so it was merging them into a copy of the app-wide output aggregator.

While I was at it, I added a better comment above the "Merge" code and
made it a bit more efficient, after learning more details about
`emplace` vs. `insert` or `operator[]` on `std::map`'s.

Co-authored-by: Kevin Frei <freik@meta.com>

Author: kevinfrei

llvm/include/llvm/DebugInfo/GSYM/OutputAggregator.h

@@ -57,13 +57,12 @@ class OutputAggregator {
   }
 
   // For multi-threaded usage, we can collect stuff in another aggregator,
-  // then merge it in here
+  // then merge it in here. Note that this is *not* thread safe. It is up to
+  // the caller to ensure that this is only called from one thread at a time.
   void Merge(const OutputAggregator &other) {
     for (auto &&[name, count] : other.Aggregation) {
-      auto it = Aggregation.find(name);
-      if (it == Aggregation.end())
-        Aggregation.emplace(name, count);
-      else
+      auto [it, inserted] = Aggregation.emplace(name, count);
+      if (!inserted)
         it->second += count;
     }
   }

llvm/lib/DebugInfo/GSYM/DwarfTransformer.cpp

@@ -612,7 +612,7 @@ Error DwarfTransformer::convert(uint32_t NumThreads, OutputAggregator &Out) {
       DWARFDie Die = getDie(*CU);
       if (Die) {
         CUInfo CUI(DICtx, dyn_cast<DWARFCompileUnit>(CU.get()));
-        pool.async([this, CUI, &LogMutex, Out, Die]() mutable {
+        pool.async([this, CUI, &LogMutex, &Out, Die]() mutable {
           std::string storage;
           raw_string_ostream StrStream(storage);
           OutputAggregator ThreadOut(Out.GetOS() ? &StrStream : nullptr);