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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I'm really liking the direction here -- this was a much more simple solution than I was expecting. Aside from some tiny nits, this looks good to me, but I leave it to Erich for the final sign-off.

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