[bazel] Introduce config setting to enable clang proto fuzzer targets

[slackito]

This change puts behind a bazel custom flag
(--@llvm-project//clang:enable_proto_fuzzer) the targets that would be enabled by the CLANG_ENABLE_PROTO_FUZZER cmake variable. This bazel flag can be specified in the command line, or added to the user's bazelrc file.

[rupprecht]

Mechanically this gets the job done, but I'd like to understand more about why we need to do this. This adds complexity that I'd rather avoid unless it's necessary.

From #123126, the request is to be able to not build these targets. But why? For example, if someone is building //clang:all and they don't want this target, the solution should be to list the specific clang tools to build.

@chapuni, can you clarify why you need these targets excluded? What does your build invocation look like?

[chapuni]

@rupprecht I'd like to use //clang:all for my private builder not to miss newly added targets.
That said, protobuf stuff is not in my scope. I don't want to append rules in my WORKSPACE for that.

For me, it'd be better if I could specify like -//clang/clang-fuzzer/....

protobuf doesn't prevent my build when I build "clang toolchains" with the explicit target. So, easy exclusions would not be necessary if you don't want to involve complexity at the moment.