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

[Erich Keane via cfe-commits]

erichkeane wrote:

Also, please don't 'force-push', just push an additional commit to your review.  It erases history and makes reviewing it about 3x harder since history/context gets lost.

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