clang-tidy: add IgnoreMacros option to modernize-use-default-member-init
And also enable it by default to be consistent with e.g.
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
Differential Revision: https://reviews.llvm.org/D32945