[clang] [clang] Reject initializing constexpr struct/union types with compound literals that contain non null pointers (PR #228776)

[Abdulmalek Almkainzi via cfe-commits]

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aalmkainzi wrote:

I tried adding it to C/C23/n3018.c but no errors happen there because of `-fexperimental-new-constant-interpreter`, same with `Sema/constexpr.c`.

I moved the existing test to `Sema/`

https://github.com/llvm/llvm-project/pull/228776