[clang] [clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854) (PR #96301)

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cor3ntin wrote:

Neither GCC, EDG nor MSVC agree with us https://godbolt.org/z/Wqrhnz63a
The behavior of GCC is a bit weird but I am pretty sure the last two tests should pass
@hubert-reinterpretcast 

https://github.com/llvm/llvm-project/pull/96301