[clang-tools-extra] [clang-tidy] Add user-defined functions to `bugprone-unsafe-functions` check (PR #106350)

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Discookie wrote:

Internal was the wrong word there, the distinction is only about whether AnnexK is available or not, in the context of the checked code. We certainly could eliminate `CustomAnnexKFunctions`, but that would mean that it's the user's responsibility to check whether AnnexK is available, outside of the code being checked.
Ultimately I don't think it would get too much use, if for nothing other than being able to reproduce the default-checked functions with just the `Custom__Functions`.

https://github.com/llvm/llvm-project/pull/106350