Untangle `GenericTypeHandler` cross-references + more IWYU in repeated_ptr_field.*

* `GenericTypeHandler` used to be forward-declared in message_lite.h and unncessarily friended by `MessageLite`, which was its only use in that source. repeated_ptr_field.h relied on that forward declaration being there, and fully defined `GenericTypeHandler` below the first use.
* `StringPieceField` was forward-declared, and now IWYU'ed.

PiperOrigin-RevId: 672686570

src/google/protobuf/message_lite.h

@@ -225,9 +225,6 @@ class WireFormatLite;
 class WeakFieldMap;
 class RustMapHelper;
 
-template <typename Type>
-class GenericTypeHandler;  // defined in repeated_field.h
-
 // We compute sizes as size_t but cache them as int.  This function converts a
 // computed size to a cached size.  Since we don't proceed with serialization
 // if the total size was > INT_MAX, it is not important what this function
@@ -959,8 +956,6 @@ class PROTOBUF_EXPORT MessageLite {
 
   template <typename Type>
   friend class Arena::InternalHelper;
-  template <typename Type>
-  friend class internal::GenericTypeHandler;
 
   friend auto internal::GetClassData(const MessageLite& msg);
 

src/google/protobuf/repeated_ptr_field.h

@@ -94,6 +94,10 @@ struct ArenaOffsetHelper {
   static constexpr size_t value = offsetof(T, arena_);
 };
 
+// Defined further below.
+template <typename Type>
+class GenericTypeHandler;
+
 // This is the common base class for RepeatedPtrFields.  It deals only in void*
 // pointers.  Users should not use this interface directly.
 //