[bazel] Add rules for clang-fuzzer protobuf-related libraries

[slackito]

Also bumped up bazel_skylib to the latest version because the proto rules were complaining about a missing feature.

[chapuni]

Could we split out clang-proto-fuzzer stuff into the directory (for me to exclude them easily)?

For now, I have:

    "-@llvm-project//clang:clang-fuzzer-initialize",
    "-@llvm-project//clang:clang-proto-to-cxx",
    "-@llvm-project//clang:cxx_cc_proto",
    "-@llvm-project//clang:cxx-proto",
    "-@llvm-project//clang:proto-to-cxx-lib",

[slackito]

Could we split out clang-proto-fuzzer stuff into the directory (for me to exclude them easily)?

I'm not opposed to this, but if these targets are causing you problems I'd like to know more details. I tested this on a machine without protoc installed so I assumed it'd be okay.

Does it need to be a separate directory or would something like a config_setting with a custom flag work for you? I think that would be similar in spirit to the CLANG_ENABLE_PROTO_FUZZER cmake flag. Then we could have a setting with a custom --@llvm-project//clang:enable_proto_fuzzer (or something like this, I'm not super familiar with this part of bazel yet) flag, that people could add to their blazerc.

For now, I have:

    "-@llvm-project//clang:clang-fuzzer-initialize",
    "-@llvm-project//clang:clang-proto-to-cxx",
    "-@llvm-project//clang:cxx_cc_proto",
    "-@llvm-project//clang:cxx-proto",
    "-@llvm-project//clang:proto-to-cxx-lib",

Do you mean only the proto-related stuff, or everything clang-fuzzer? If it's the former, we don't need to exclude clang-fuzzer-initialize because it doesn't depend on protobufs. And if it's the latter, we'd also need to exclude/move some other recently-added targets like clang-fuzzer-dictionary and handle-cxx.

[chapuni]

Do you mean only the proto-related stuff, or everything clang-fuzzer? If it's the former, we don't need to exclude clang-fuzzer-initialize because it doesn't depend on protobufs. And if it's the latter, we'd also need to exclude/move some other recently-added targets like clang-fuzzer-dictionary and handle-cxx.

Thanks for your considerations.
Practically the former but it'd be better able to exclude stuff controlled by CLANG_ENABLE_PROTO_FUZZER.

I am using @llvm-project for bootstrapping clang toolchain. clang-fuzzer is not required for now.
My builder has the stage for comparison of each relocatable-object.o file CMake tree and Bazel tree but I want to build much target as possible. (Note, almost all files can be made identical with my toolchain and toolchain's clang can be made identical to the final artifact clang)

I don't use the in-tree WORKSPACE. Ultimately I will have to configure protobuf but for now I would like to keep minimal configuration.

[slackito]

I think something like #123833 should do the trick.