[clang-tidy] modernize-use-default default constructor bugfix

Summary:
Only member initializers that are written should prevent
using '= default' on a default constructor.

Reviewers: klimek, sbenza, aaron.ballman, alexfh

Subscribers: Eugene.Zelenko, alexfh, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D24444

llvm-svn: 283869