[llvm] [SECURITY.md] add pointers to "what is a security issue" (PR #177372)

[Kristof Beyls via llvm-commits]

https://github.com/kbeyls approved this pull request.

I think this looks great; I can't come up with suggestions for how the text could be improved further ;)

https://github.com/llvm/llvm-project/pull/177372