clang-tidy: add IgnoreMacros option to modernize-use-default-member-init

Summary:
And also enable it by default to be consistent with e.g.
modernize-use-using.

This helps e.g. when running this check on cppunit client code where the
macro is provided by the system, so there is no easy way to modify it.

Reviewers: alexfh, malcolm.parsons

Reviewed By: malcolm.parsons

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D32945

llvm-svn: 302429