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/generic.cpp

@@ -532,4 +532,17 @@ auto GetInstForSpecific(Context& context, SemIR::SpecificId specific_id)
   }
 }
 
+auto DiagnoseIfGenericMissingExplicitParameters(
+    Context& context, SemIR::EntityWithParamsBase& entity_base) -> void {
+  if (!entity_base.implicit_param_patterns_id.has_value() ||
+      entity_base.param_patterns_id.has_value()) {
+    return;
+  }
+
+  CARBON_DIAGNOSTIC(GenericMissingExplicitParameters, Error,
+                    "expected explicit parameters after implicit parameters");
+  context.emitter().Emit(entity_base.last_param_node_id,
+                         GenericMissingExplicitParameters);
+}
+
 }  // namespace Carbon::Check