[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.

This is an amended followup to https://reviews.llvm.org/D57207

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: jdoerfert, xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58095

llvm-svn: 354534