[clang-tidy] Ignore implicit casts in modernize-use-default-member-init

Summary:
Initialising a pointer from nullptr involves an implicit cast.
Ignore it after getting initialiser from InitListExpr.

Fixes: PR44440

Reviewers: aaron.ballman, alexfh, JonasToth

Reviewed By: JonasToth

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72630