[clang-tidy] cppcoreguidelines-interfaces-global-init

Summary:
This check flags initializers of globals that access extern objects, and therefore can lead to order-of-initialization problems (this recommandation is part of CPP core guidelines).
Note that this only checks half of the guideline for now (it does not enforce using constexpr functions).

Reviewers: aaron.ballman, alexfh

Subscribers: aaron.ballman, etienneb, Eugene.Zelenko, cfe-commits

Patch by Clement Courbet!

Differential Revision: http://reviews.llvm.org/D18649

llvm-svn: 265774