Let GeneratedMessageTraits use C++17 feature of auto template arguments.

This is mostly a style fix so that we can avoid using decltype which is a bit
cumbersome. auto template arguments is a C++17 feature so we can only do this
after we've migrated the buildchain to use C++17.

We upgraded main branch to C++17 in commit
fe53593

PiperOrigin-RevId: 706015756

src/google/protobuf/compiler/cpp/file.cc

@@ -1503,9 +1503,8 @@ class FileGenerator::ForwardDeclarations {
         if (options.dllexport_decl.empty()) {
           p->Emit(R"cc(
             template <>
-            internal::GeneratedMessageTraitsT<
-                decltype($default_name$), &$default_name$,
-                decltype($class$_class_data_), &$class$_class_data_>
+            internal::GeneratedMessageTraitsT<&$default_name$,
+                                              &$class$_class_data_>
                 internal::MessageTraitsImpl::value<$class$>;
           )cc");
         }

src/google/protobuf/message_lite.h

@@ -242,9 +242,7 @@ class PROTOBUF_EXPORT CachedSize {
 
 auto GetClassData(const MessageLite& msg);
 
-// TODO: Upgrade to `auto` parameters when we drop C++14 support.
-template <typename T, const T* kDefault, typename ClassData,
-          const ClassData* kClassData>
+template <const auto* kDefault, const auto* kClassData>
 struct GeneratedMessageTraitsT {
   static constexpr const void* default_instance() { return kDefault; }
   static constexpr const auto* class_data() { return kClassData->base(); }