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_interface.cpp

@@ -62,6 +62,8 @@ static auto BuildInterfaceDecl(Context& context,
       name, interface_decl_id, /*is_extern=*/false,
       SemIR::LibraryNameId::None)};
 
+  DiagnoseIfGenericMissingExplicitParameters(context, interface_info);
+
   // Check whether this is a redeclaration.
   SemIR::ScopeLookupResult lookup_result =
       context.decl_name_stack().LookupOrAddName(