[clang] [Clang] allow [[msvc::constexpr]] usage outside the std namespace (PR #119153)

[Richard Dzenis via cfe-commits]

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RIscRIpt wrote:

Instead of adding this new test, could you please modify behavior of existing test: [`clang/test/SemaCXX/ms-constexpr-new.cpp`](https://github.com/llvm/llvm-project/pull/71300/files#diff-b0e33c7c54dd10b988b2badcc60be091e69ff3aeaf86c95cb2bb9bec14ac376f).



https://github.com/llvm/llvm-project/pull/119153