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

[Erich Keane via cfe-commits]

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@@ -235,6 +235,7 @@ Non-comprehensive list of changes in this release
   except that it returns the size of a type ignoring tail padding.
 * ``__builtin_classify_type()`` now classifies ``_BitInt`` values as the return value ``18``
   and vector types as return value ``19``, to match GCC 14's behavior.
+* Since MSVC 19.33 added undocumented attribute ``[[msvc::constexpr]]``, this release adds the attribute as well.
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erichkeane wrote:

I didn't intend for you to remove the first bit of this, which I think still added value:

`The default value of `_MSC_VER` was raised from 1920 to 1933.`

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