[llvm] [bazel] Introduce config setting to enable clang proto fuzzer targets (PR #123833)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

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?

https://github.com/llvm/llvm-project/pull/123833