[clang] [C23] Implement N3018: The constexpr specifier for object definitions (PR #73099)

[Mariya Podchishchaeva via cfe-commits]

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+// RUN: %clang_cc1 -std=c2x -verify -triple x86_64 -pedantic -Wno-conversion -Wno-constant-conversion %s
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Fznamznon wrote:

Done.

https://github.com/llvm/llvm-project/pull/73099