[clang-tidy] cppcoreguidelines-pro-type-member-init should not complain about static variables

Summary:
Variables with static storage duration are zero-initialized per
[stmt.dcl]p4 and [basic.start.init]p2.

Reviewers: sbenza, aaron.ballman

Subscribers: michael_miller, flx, cfe-commits

Differential Revision: http://reviews.llvm.org/D19672

llvm-svn: 267933