[libc++] Adds __cpp_lib_three_way_comparison FTM. (#91515)

The paper
  P0768R1 Library Support for the Spaceship (Comparison) Operator
did not add a feature-test macro. This omission has been corrected in
  P1353R0 Missing Feature Test Macros

This enables the FTM for P0768R1

Fixes: https://github.com/llvm/llvm-project/issues/73953



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Co-authored-by: S. B. Tam <cpplearner@outlook.com>