[clang] [NFC][Clang] Add test for P2843R3 - Preprocessing is never undefined (PR #192073)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

LGTM but please remove the `NFC` tag from the patch summary as this does have functional changes for C++26 mode.

https://github.com/llvm/llvm-project/pull/192073