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

[Richard Dzenis via cfe-commits]

https://github.com/RIscRIpt commented:

Addressed review comments; updated my tests to use changed diagnostic messages.

Next I'll upload my branch rebased onto main.

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