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

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Some bots are not happy with one of the tests. I contacted you on Discord about your next steps.

I'll have to revert the patch if I won't hear from you soon.

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