[clang] [Clang] [Sema] Error on reference types inside a union with msvc 1900+ (PR #102851)

[Reid Kleckner via cfe-commits]

rnk wrote:

Something to consider is that `-fms-extensions` has, at various points, been used as a tool for making Windows code portable to Apple platforms. So, there may be users out there who use references in unions on non-Windows platforms. However, they presumably also build with modern versions of MSVC, which dropped this extension long ago.

My point is, with this in mind, this seems release-note-worthy, but otherwise, we can just do this.

https://github.com/llvm/llvm-project/pull/102851