[clang] [clang-cl] [Sema] Support MSVC non-const lvalue to user-defined temporary reference (PR #99833)

[Max Winkler via cfe-commits]

MaxEW707 wrote:

> I'd like to see this change make it into the repo, as I'm in a similar situation with a 6+ million line codebase. What's the next steps to advance this?

Hey sorry. I've been busy with other work prios and an msvc mangling bug. I still intend to get back to this as I also need it.
I plan to get back to it in about 1.5 weeks to address all the feedback.

I also want to change it to be opt-in for clang-cl for now since it is the first impl instead of being opt-out like in msvc.
I don't want to accidentally break users who are using clang-cl without `/permissive-` especially since it changes overload resolution behaviour.

https://github.com/llvm/llvm-project/pull/99833