[clang-tidy] fix modernize-use-nullptr false positive with spaceship operator comparisons

`clang-tidy -std=c++20` with `modernize-use-nullptr` mistakenly inserts `nullptr` in place of the comparison operator if the comparison internally expands in the AST to a rewritten spaceship operator. This can be reproduced by running the new `modernize-use-nullptr-cxx20.cpp` test without applying the supplied patch to `UseNullptrCheck.cpp`; the current clang-tidy will mistakenly replace:
```result = (a1 < a2);```
with
```result = (a1 nullptr a2);```

Reviewed By: njames93

Differential Revision: https://reviews.llvm.org/D95714