[clang-tidy] Make google-objc-function-naming ignore implicit functions 🙈

Summary:
Implicit functions are outside the control of source authors and should
be exempt from style restrictions.

Tested via running clang tools tests.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D57207

llvm-svn: 352968