[clang] [Clang] Remove __is_nullptr (PR #99038)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I mostly support the removal but I'd feel more comfortable documenting the extension as deprecated in Clang 18, adding a release note warning users about it going away and what the replacement code would look like, and then remove `__is_nullptr()` in Clang 19.

That said, has anyone checked with Embarcadero and Microsoft as to whether they still need it supported for their STL implementations for historical reasons? That might delay the timeline or change opinions on the direction.

https://github.com/llvm/llvm-project/pull/99038