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

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Discookie wrote:

Note: I copied over the header from the test `unsafe-functions.c` about the tests not working on some targets into my new tests. How could I test these files on the targets? (Other than just pushing this branch I suppose.)

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