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

[Kristóf Umann via cfe-commits]

Szelethus 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.

I'm still not sure why we need both `CustomNormalFunctions` and `CustomAnnexKFunctions`. From what I understand, this is due  to something specific in implementation, but why is it the user that needs to bear that burden? Can we eliminate one of these options?

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