kernel/hle_ipc: Convert std::shared_ptr IPC header instances to std::optional

There's no real need to use a shared lifetime here, since we don't
actually expose them to anything else. This is also kind of an
unnecessary use of the heap given the objects themselves are so small;
small enough, in fact that changing over to optionals actually reduces
the overall size of the HLERequestContext struct (818 bytes to 808
bytes).

src/core/hle/ipc_helpers.h

@@ -350,7 +350,7 @@ public:
     template <class T>
     std::shared_ptr<T> PopIpcInterface() {
         ASSERT(context->Session()->IsDomain());
-        ASSERT(context->GetDomainMessageHeader()->input_object_count > 0);
+        ASSERT(context->GetDomainMessageHeader().input_object_count > 0);
         return context->GetDomainRequestHandler<T>(Pop<u32>() - 1);
     }
 };