[clang] [clang] Stop claiming __builtin_assume_separate_storage is constexpr (PR #225515)

[Mariya Podchishchaeva via cfe-commits]

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@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
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Fznamznon wrote:

We need to test the new evaluator too, especially if this test evolves when the builtin is supported in constexpr
```suggestion
// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify -fnew-experimental-constant-interpreter %s
```

https://github.com/llvm/llvm-project/pull/225515