[clang-tidy] Handle bitfields in cppcoreguidelines-pro-type-member-init if using C++2a

Summary:
C++2a allows bitfields to have default member initializers.
Add support for this to clang-tidy's cppcoreguidelines-pro-type-member-init check.

Reviewers: aaron.ballman, alexfh

Reviewed By: aaron.ballman

Subscribers: klimek, nemanjai, xazax.hun, kbarton, cfe-commits

Differential Revision: https://reviews.llvm.org/D42426

llvm-svn: 323227