[clang-tidy] Ignore default arguments in modernize-default-member-init

Summary:
Default member initializers cannot refer to constructor parameters, but modernize-default-member-init was trying to when the default constructor had default arguments.

Change the check to ignore default arguments to the default constructor.

Fixes PR31524.

Reviewers: alexfh, aaron.ballman

Subscribers: cfe-commits, JDevlieghere, Eugene.Zelenko

Differential Revision: https://reviews.llvm.org/D28287

llvm-svn: 290972