[clang-tidy] New check to warn when storing dispatch_once_t in non-static, non-global storage.

Summary:
Creates a new darwin ClangTidy module and adds the darwin-dispatch-once-nonstatic check that warns about dispatch_once_t variables not in static or global storage. This catches a missing static for local variables in e.g. singleton initialization behavior, and also warns on storing dispatch_once_t values in Objective-C instance variables. C/C++ struct/class instances may potentially live in static/global storage, and are ignored for this check.

The osx.API static analysis checker can find the non-static storage use of dispatch_once_t; I thought it useful to also catch this issue in clang-tidy when possible.

This is a re-land of https://reviews.llvm.org/D67567

Reviewers: thakis, gribozavr, stephanemoore

Subscribers: Eugene.Zelenko, mgorny, xazax.hun, jkorous, arphaman, kadircet, usaxena95

Tags: #clang-tools-extra, #clang, #llvm

Differential Revision: https://reviews.llvm.org/D68109

llvm-svn: 373065