Fix parse support for 'fn F[];' (#5135)

According to approved syntax at
https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3848.md#syntax-defined,
`fn F[]` without explicit parameters should be valid. This makes it
work, then adds some validation to prevent `class C[]` in check.

Note that for `fn`, positional parameters are a TODO -- but this allows
me to test validation in `fn destroy[]` which is rejected, not just a
TODO.

toolchain/check/handle_class.cpp

@@ -233,6 +233,8 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
        .self_type_id = SemIR::TypeId::None,
        .inheritance_kind = inheritance_kind}};
 
+  DiagnoseIfGenericMissingExplicitParameters(context, class_info);
+
   MergeOrAddName(context, node_id, name_context, class_decl_id, class_decl,
                  class_info, is_definition,
                  introducer.modifier_set.GetAccessKind());