[clang] Thread safety analysis: provide printSCFG definition. (PR #80277)

[Haojian Wu via cfe-commits]

hokein wrote:

> It might have been commented out so that it doesn't take up space in the compiled binary.
> 
> I'd like seeing it compiled, just to make sure it doesn't break. But I'd also like if it doesn't appear in the final binary. Perhaps we can change visibility so that `--gc-sections` or LTO can optimize it out? But I'm not sure if that's enough.

If we don't want it to be in the final binary, how about guarding this part of code (`declaration` & `definition`) with `#ifndef NDEBUG`? 

https://github.com/llvm/llvm-project/pull/80277