[ffigen] Remove Writer.usedEnumCTypes

[liamappelbe]

Part of a larger refactor where I'm trying to make Writer less stateful during code gen. Currently there's a lot of stuff that should be implemented using visitors that is actually tracked by a bit of Writer state during codegen. End goal is to make the type to string conversion functions take Context instead of Writer.

usedEnumCTypes is only used to generate a warning message when enums are passed through FFI APIs, because doing so is unsafe (different C compilers treat enums differently).

The old logic added the enum to usedEnumCTypes when its getCType method was called during code generation. The enum's own generated bindings don't use getCType, only other bindings that refer to the enum call this method (eg when passing an enum as a function param). So this was a roundabout (and brittle) way of checking if the enum is used in any APIs. If we later decided to use getCType in the enum's own bindings, this check would start producing a ton of false positives.

A more direct approach is just to run a visitor from all non-enum bindings and see if we find any enums.

Part of #2462 and #1259
This is also yak shaving for #2419
The CI failure is unrelated: #2517

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 91.825% (+0.1%) from 91.701%
when pulling 9cfce59 on ffigen_less_writer
into dc4aa33 on main.