Fix use-after-free found by ASAN

DenseMap::lookup returns copy of the value in the map. Returning the
address of the temporary return value will cause use-after-free.

llvm-svn: 295675

llvm/include/llvm/Support/DebugCounter.h

@@ -111,7 +111,7 @@ public:
   unsigned int getNumCounters() const { return RegisteredCounters.size(); }
 
   // Return the name and description of the counter with the given ID.
-  std::pair<StringRef, StringRef> getCounterInfo(unsigned ID) const {
+  std::pair<std::string, std::string> getCounterInfo(unsigned ID) const {
     return std::make_pair(RegisteredCounters[ID], CounterDesc.lookup(ID));
   }