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

[via cfe-commits]

Discookie wrote:

`CustomAnnexKFunctions` exists because the checker has two different internal matching modes for whether AnnexK is enabled or not. This both depends on `__STDC_LIB_EXT1__` (defined by system) `__STDC_WANT_LIB_EXT1__` (defined by the user), so it's reasonable to expect that a project could have differing availability of AnnexK.

I could rename CustomNormalFunctions to CustomFunctions or something to make it clearer that it's always available.

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