[clang-tools-extra] [clang-tidy] Add a fully custom message to `bugprone-unsafe-functions` (PR #162443)

[via cfe-commits]

Discookie wrote:

No particular reason for choosing the prefix `>`. I thought about enclosing it in `"` quotes, with escape sequences like `\"`, but that would require implementing a parser in clang-tidy, and that seemed to be unnecessarily big in scope. A single-character prefix seemed better than that.

I'll rephrase the docs a bit to make it clearer.

https://github.com/llvm/llvm-project/pull/162443