[clang] [clang-cl] Add support for [[msvc::constexpr]] C++11 attribute (PR #71300)

[Erich Keane via cfe-commits]

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@@ -217,6 +217,8 @@ C23 Feature Support
 
 Non-comprehensive list of changes in this release
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+- The default value of `_MSC_VER` was raised from 1920 to 1933.
+  MSVC 19.33 added undocumented attribute ``[[msvc::constexpr]]``.
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erichkeane wrote:

```suggestion
  Since MSVC 19.33 added undocumented attribute ``[[msvc::constexpr]]``, this release adds the attribute as well.
```

https://github.com/llvm/llvm-project/pull/71300