[clang] [Clang] [Sema] Error on reference types inside a union with msvc 1910+ (PR #102851)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I definitely remember this being an extension in older versions of VS around VS 2012 but don't know when MSVC no longer exactly removed support for this extension wholesale.

This was definitely required for MFC headers; I remember implementing this horrible nonsense ages ago, see https://github.com/llvm/llvm-project/issues/14109 -- I can verify that `CComControlBase` no longer has the terrible union in it, but I'm not certain when it lost it.

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