Start using clang-cl for Windows Bazel build

With this commit we start using 'clang-cl' to build Mozc for Windows
with Bazel.

While the original motivation of switching to 'clang-cl' is that
protobuf is planning to stop supporting cl.exe with bazel build in
protobuf v34 [1], but there is also an on-going build breakage that
happens when combining protobuf and abseil-cpp 20250127.0, which is not
yet fixed [2]. The latter one is more critical for us because more and
more bzlmod dependencies start requiring abseil-cpp 20250127.0 or
higher.

There must be no impact on Windows GYP build.

Closes google#1179.

 [1]: protocolbuffers/protobuf#20085
 [2]: protocolbuffers/protobuf#20331

Author: yukawa

src/tools/README.md

@@ -0,0 +1,5 @@
+# tools
+
+This file contains a special [wrapper script for bazelisk](https://github.com/bazelbuild/bazelisk/blob/master/README.md#toolsbazel).
+
+ * [bazel.bat](./bazel.bat)

src/tools/bazel.bat

@@ -0,0 +1,7 @@
+@echo off
+SET selfpath=%~dp0
+set BAZEL_LLVM=%selfpath:~0,-6%\third_party\llvm\clang+llvm-19.1.7-x86_64-pc-windows-msvc
+%BAZEL_REAL% %* & call:myexit
+
+:myexit
+exit /b

src/windows.bazelrc

@@ -7,9 +7,8 @@
 #   bazel --bazelrc=windows.bazelrc build //protocol:commands_proto \
 #         --config oss_windows
 
-# There has been a long standing path-length issue on building protobuf on
-# Windows with bazel. See the following for details.
-# https://github.com/protocolbuffers/protobuf/issues/12947
-# https://github.com/bazelbuild/bazel/issues/18683
-# Here is an ugly workaround, which we really hope to get rid of.
-startup  --output_base=C:/x --windows_enable_symlinks
+startup  --windows_enable_symlinks
+
+build    --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl
+build    --extra_toolchains=@local_config_cc//:cc-toolchain-x64_x86_windows-clang-cl
+build    --host_platform=//:host-windows-clang-cl