[clang] [Clang] Add `std:c++26preview` for MSVC compat (PR #207693)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

LGTM!

I was wondering why we were adding a flag that MSVC doesn't currently support... from the linked comment:

> If anyone complains that Clang added a switch that MSVC doesn't have yet, blame me. It's all my fault!!

That explains it. It's STL's fault. :-D

https://github.com/llvm/llvm-project/pull/207693