`clang-cl` toolchain for x86 (32-bit) binaries is missing

## Description of the problem:

As the protobuf team is planning to stop supporting the combination of Bazel and `cl.exe` and start recommending `clang-cl.exe` as the alternative, our project (https://github.com/google/mozc) is reviewing what are potential brokers when migrating from `cl.exe` to `clang-cl.exe`.

* https://github.com/protocolbuffers/protobuf/issues/20085

One thing I noticed is that `rules_cc` as of 0.0.17 doesn't look to offer a `clang-cl` toolchain to build x86 (32-bit) binaries.

https://github.com/bazelbuild/rules_cc/blob/a1162270a0bb680190e8b4f3dab066f15a1ede6c/cc/private/toolchain/BUILD.windows.tpl#L73-L98

It would be really nice if `rules_cc` can provide a `clang-cl` toolchain to build x86 (32-bit) binaries.