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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I think this is pretty close, I think skipping on constructors for now is acceptable.  However, i think we should enable the test, and record the diagnostic to ensure we show it is invalid for now (along with better showing the TODO).

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