[clang-tidy] Add the `bugprone-unsafe-functions` check

Checks for unsafe functions, mostly those listed in the
SEI CERT C Coding Standard Recommendation `MSC24-C` and Rule `MSC33-C`.

For the listed functions, an alternative, more secure replacement is
suggested, if such is available. The checker heavily relies on the
functions from "Annex K" (Bounds-checking interfaces) from C11, but
there are several other recommendations not directly from Annex K.

Differential Revision: http://reviews.llvm.org/D91000



Reviewed-By: aaron.ballman, dkrupp, steakhal, whisperity

Co-Authored-By: Tamás Koller <koller.tamas1996@gmail.com>
Co-Authored-By: Balázs Benics <balazs.benics@sigmatechnology.se>
Co-Authored-By: Whisperity <whisperity@gmail.com>