[llvm] [llvm-c] Guard include of llvm-config in Visibility.h (PR #154229)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

> The problem is that the configuration is not LLVMC, but LLVM itself. You need the configuration for the target libraries that it is creating bindings for.

I think you are trying to solve a bigger problem than the one trying to fix here. For this PR, just want to achieve the same visibility granularity as `main` while try to break `llvm-c` out of the dependency of the big config file for c++ header.

I don't know how people usually use llvm-c headers usually. I don't assume people will use c++ and c header together. I might consider split out all the functions that implements C interface into a separate lib `libLLVMC` and make that link other c++ libraries. 

https://github.com/llvm/llvm-project/pull/154229