Fix indirect import handling for functions. (#4258)

The particular test this focused on is indirect_two_file in
toolchain/check/testdata/function/definition/no_prelude/extern_library.carbon.

This removes `parent_scope_id_for_new_inst` because I think it's
returning unhelpful results. The use was at the root of incorrect
results for the indirect import chain. `name_id_for_new_inst` is
actually wrapping a union, so it's more important.

The merging of `is_extern` and `first_owning_decl_id` in
`handle_function.cpp` feels like it's less correct with the changes
that've been made to `extern`. This ripples in tests, because the error
recovery shifts.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_namespace.cpp

@@ -70,7 +70,7 @@ auto HandleParseNode(Context& context, Parse::NamespaceId node_id) -> bool {
   if (!namespace_inst.name_scope_id.is_valid()) {
     namespace_inst.name_scope_id = context.name_scopes().Add(
         namespace_id, name_context.name_id_for_new_inst(),
-        name_context.parent_scope_id_for_new_inst());
+        name_context.parent_scope_id);
   }
 
   context.ReplaceInstBeforeConstantUse(namespace_id, namespace_inst);