[clang] [clang] Allow C-style casts in constexpr in MS compatible mode (PR #197005)

[via cfe-commits]

eleviant wrote:

>Can we maybe add a member "SmallVectorImpl *ExtensionDiag" to EvalStatus, and make the callers print the diagnostics?

@efriedma-quic Well, I finally decided to try this out (check the latest update). The main motivation was that with `-fms-compatibility` we do in fact bypass not just reinterpret_cast/C-style cast but also taking field address of a null pointer. I've made separate warning messages for both of these cases, which are now being emitted at call sites.

https://github.com/llvm/llvm-project/pull/197005