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

[Richard Dzenis via cfe-commits]

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+// RUN: %clang_cc1 -fms-compatibility -fms-compatibility-version=19.33 -std=c++20 -verify=supported %s
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RIscRIpt wrote:

We can just remove `__cdecl` altogether. I put it there to replicate Microsoft's STL. MSVC accepts that test case without `__cdecl` too.

Regarding other compiler error, it was fixed in 3ec6c72551846b8f4143c8c101a1a6203e85a2aa

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