[clang-tidy] Handle unions with existing default-member-init

Summary:
clang-tidy's modernize-use-default-member-init was crashing for unions
with an existing default member initializer.

Fixes PR40492

Reviewers: aaron.ballman, alexfh, JonasToth

Reviewed By: JonasToth

Subscribers: JonasToth, riccibruno, xazax.hun, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D57665

llvm-svn: 353092