[libc++] Clang-tidy enable modernize-use-nullptr.

Clang-tidy 18 no longer has false positives with the spaceship operator.
Note that I'm quite sure there are more occurrences in our headers that
are not caught.

This relands https://github.com/llvm/llvm-project/pull/76659 with fixes
tested in https://github.com/llvm/llvm-project/pull/78746.