Handle "gaps" in C++ vtables. (#7206)

Not every entry in a C++ vtable corresponds to a function that we want
to import. For the holes, leave a `SemIR::InstId::None` in the vtable.
Also mark vtables that extend a C++ vtable as being non-Carbon-native so
we don't try to lower them (and crash on the `None` entries).

In particular, we leave holes for destructors, since we don't have
destructor declarations on the Carbon side that need to override them.

toolchain/check/import_ref.cpp

@@ -2454,6 +2454,9 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
   for (auto [import_vtable_entry_inst_id, local_vtable_entry_inst_id] :
        llvm::zip_equal(virtual_functions, lazy_virtual_functions)) {
+    if (!local_vtable_entry_inst_id.has_value()) {
+      continue;
+    }
     // Use LoadedImportRef for imported symbolic constant vtable entries so they
     // can carry attached constants necessary for applying specifics to these
     // constants when they are used.