[clang-tidy] Handle bitfields in cppcoreguidelines-pro-type-member-init

Summary:
Unnamed bitfields cannot be initialized.
Bitfields cannot be in-class initialized.

Reviewers: alexfh, hokein, aaron.ballman

Subscribers: Prazek, nemanjai, cfe-commits

Differential Revision: https://reviews.llvm.org/D26119

llvm-svn: 285752