Introduce platform host-windows-clang-cl

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 alphabeticall 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.

src/BUILD.bazel

@@ -97,6 +97,16 @@ selects.config_setting_group(
     visibility = ["//:__subpackages__"],
 )
 
+# Generic Windows platform with clang-cl that automatically matches host CPU.
+platform(
+    name = "host-windows-clang-cl",
+    constraint_values = [
+        "@platforms//os:windows",
+        "@bazel_tools//tools/cpp:clang-cl",
+    ],
+    parents = ["@local_config_platform//:host"],
+)
+
 # Special target so as to define special macros for each platforms.
 # Don't depend on this directly. Use mozc_cc_(library|binary|test) rule instead.
 cc_library(