Support for initialization of classes with abstract base classes. (#6802)

When initializing `.base` in class initialization, use `partial Base` as
the destination type rather than `Base`. Treat `partial Base` as not
being abstract even when `Base` is.

Allow conversion from a `partial T` initializer to a `T` initializer.
Store the vptr while performing the conversion. Do not store the vptr
when performing a `partial T` initialization, only when performing a
non-partial `T` initialization.

toolchain/sem_ir/expr_info.cpp

@@ -178,6 +178,13 @@ auto FindStorageArgForInitializer(const File& sem_ir, InstId init_id,
         init_id = init.result_id;
         continue;
       }
+      case CARBON_KIND(UpdateInit init): {
+        if (!allow_transitive) {
+          return InstId::None;
+        }
+        init_id = init.base_init_id;
+        continue;
+      }
       case CARBON_KIND(ArrayInit init): {
         return init.dest_id;
       }