[native_toolchain_c] Support Clang on Windows 2

[dcharkes]

Closes: #1892

Using clang on Windows requires being run with the windows dev environment or an MSVC toolchain installed which is discoverable on the system.

On the GitHub bots, MSVC comes pre-installed, so we can't reproduce the case where Clang cannot find the MSVC dev environment.

I've tested this locally by (1) uninstalling visual studio, (2) seeing that the build with Clang fails, (3) hardcoding in a dev environment (from depot_tools) in native_toolchain_c, and (4) seeing that that the tests pass.

[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: 90.631% (+0.002%) from 90.629%
when pulling 12ff4da on clang-windows-2
into 434db67 on main.