[clang-tidy] Handle bitfields in modernize-use-default-member-init if using C++2a

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

Reviewers: aaron.ballman, alexfh

Reviewed By: aaron.ballman

Subscribers: klimek, xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D42413

llvm-svn: 323208