Preserve the is_final bit when importing an impl declaration (#5461)

We were importing all impls as non-final, since we forgot to set the new
field when constructing the imported Impl. Adds a test that fails before
this PR, since the imported Impl is treated as non-final.

toolchain/check/import_ref.cpp

@@ -2126,7 +2126,8 @@ static auto MakeImplDeclaration(ImportContext& context,
        {.self_id = SemIR::TypeInstId::None,
         .constraint_id = SemIR::TypeInstId::None,
         .interface = SemIR::SpecificInterface::None,
-        .witness_id = witness_id}});
+        .witness_id = witness_id,
+        .is_final = import_impl.is_final}});
 
   // Write the impl ID into the ImplDecl.
   auto impl_const_id =