Windows support?

I'm using latest commit with git override:
```
bazel_dep(name = "toolchains_llvm", version = "1.1.2")
git_override(
module_name = "toolchains_llvm",
commit = "4ab573b1b87a57791ef2f9ccee71cbad80a2abb9",
remote = "https://github.com/bazel-contrib/toolchains_llvm",
)

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")
llvm.toolchain(
name = "llvm_toolchain",
llvm_version = "19.1.0",
)
use_repo(llvm, "llvm_toolchain")
register_toolchains("@llvm_toolchain//:all")
```

And for some reason:
```
ERROR: C:/users/user/_bazel_user/df37aufx/external/googletest~/BUILD.bazel:170:11: Compiling googlemock/src/gmock_main.cc failed: (Exit 2): cl.exe failed: error executing CppCompile command (from target @@googletest~//:gtest_main) C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.37.32822\bin\HostX64\x64\cl.exe @bazel-out/x64_windows-fastbuild/bin/external/googletest~/_objs/gtest_main/gmock_main.obj.params
cl : Command line error D8021 : invalid numeric argument '/Werror'
```
Works fine on linux though.

Start with the reported external/googletest~/BUILD.bazel location and the generated CppCompile command passed to cl.exe. Compare the Windows toolchain configuration with the Linux setup, then verify that the reported Google Mock compilation completes on Windows without the invalid flag error.

Written by the indexing model from the issue text.