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

[Richard Dzenis via cfe-commits]

RIscRIpt wrote:

Oof, sorry, I forgot I should push new changes and rebased changes separately (otherwise "Compare" link does not make sense). If you had checkout `829f8098af96` locally, you can see the diff diff as follows:

```
git range-diff 829f8098af96~2..829f8098af96 7d8b1c0fae42~2..7d8b1c0fae42
```

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