[clang] [C++26] Implement P2843R3 - Preprocessing is never undefined (define/undef keyword part) (PR #218322)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I don't have any problems iwth it, just a pair of comments.  I'll leave it to the others to approve this when it is ready.

https://github.com/llvm/llvm-project/pull/218322