Introduce platform host-windows-clang-cl

[yukawa]

Description

This is a preparation to build Mozc for Windows with clang-cl (#1179).

Now that Windows Bazel builds rely on CC toolchain resolution (#1112), clang-cl needs to be picked up based on platform specified to Bazel. This gives us an inverse problem what needs to be passed to Bazel so that such a resolution can happen.

The simplest solution as far as I have figures out is doing the following two steps.

1. Specify --host_platform=//:host-windows-clang-cl, where it is defined as follows.

   platform(
       name = "host-windows-clang-cl",
       constraint_values = [
           "@platforms//os:windows",
           "@bazel_tools//tools/cpp:clang-cl",
       ],
       parents = ["@local_config_platform//:host"],
   )
   

   By doing this, @bazel_tools//tools/cpp:clang-cl can be propagated into each execution platform, which is mandatory for clang-cl to be picked up.

2. Specify --extra_toolchains command line options to put the following toolchains from rules_cc in higher priority than cl.exe.

   • @local_config_cc//:cc-toolchain-x64_x86_windows-clang-cl
   • @local_config_cc//:cc-toolchain-x64_windows-clang-cl

   This is important because CC toolchain resolution picks up the first toolchain that satisfies all the constraints. To give a higher priority to clang-cl toolchains, they need to be registered before 'cl' toolchains are registered.

   Note that

   register_toolchains("@local_config_cc//:all")
   

   in MODULE.bazel registers toolchains in the alphabetical order. To give a higher priority to clang-cl toolchains, --extra_toolchains command line looks to be the best way.

What this commit does is the step 1. Without the step 2, there must be no observable behavior change yet.

Issue IDs

• See if we can use clang-cl to build Mozc for Windows with Bazel #1179

Steps to test new behaviors (if any)

• OS: Windows 11 24H2
• Steps:
  1. Install clang+llvm-19.1.7-x86_64-pc-windows-msvc.tar.xz
  2. Run the following command

     bazelisk --bazelrc=windows.bazelrc build package --config oss_windows --config release_build --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl  --extra_toolchains=@local_config_cc//:cc-toolchain-x64_x86_windows-clang-cl --host_platform=//:host-windows-clang-cl