[DWARFLinker] Avoid repeated hash lookups (NFC) (#109904)

Author: kazutakahirata

llvm/include/llvm/DWARFLinker/IndexedValuesMap.h

@@ -21,11 +21,9 @@ namespace dwarf_linker {
 template <typename T> class IndexedValuesMap {
 public:
   uint64_t getValueIndex(T Value) {
-    typename ValueToIndexMapTy::iterator It = ValueToIndexMap.find(Value);
-    if (It == ValueToIndexMap.end()) {
-      It = ValueToIndexMap.insert(std::make_pair(Value, Values.size())).first;
+    auto [It, Inserted] = ValueToIndexMap.try_emplace(Value, Values.size());
+    if (Inserted)
       Values.push_back(Value);
-    }
     return It->second;
   }