Avoid calculating the string hash twice in GsymCreator::insertString.

[simon.giesecke]

Summary

Do the single hash calculation before acquiring the lock, to reduce
lock contention. If Copy is true, and the string was not yet contained
in the StringStorage, use the new address from StringStorage, but
reuse the hash we already calculated.

[simon.giesecke]

I moved the hash calculation out of the lock now, as suggested in D102486.

[simon.giesecke]

Update using arc

llvm/lib/DebugInfo/GSYM/GsymCreator.cpp

 uint32_t GsymCreator::insertString(StringRef S, bool Copy) {
   if (S.empty())
     return 0;
+
+  // The hash can be calculated outside the lock.
+  CachedHashStringRef CHStr(S);
   std::lock_guard<std::recursive_mutex> Guard(Mutex);
   if (Copy) {
     // We need to provide backing storage for the string if requested

     // copied, but any string created by code will need to be copied.
     // This allows GsymCreator to be really fast when parsing DWARF and
     // other object files as most strings don't need to be copied.
-    CachedHashStringRef CHStr(S);
     if (!StrTab.contains(CHStr))
-      S = StringStorage.insert(S).first->getKey();
+      CHStr = CachedHashStringRef{StringStorage.insert(S).first->getKey(),
+                                  CHStr.hash()};
   }
-  return StrTab.add(S);
+  return StrTab.add(CHStr);
 }
 
 void GsymCreator::addFunctionInfo(FunctionInfo &&FI) {