Add test for spaceship operator to __config

Summary:
The libcxx test suite auto-detects spaceship operator, but __config does not. This means that the libcxx test suite has been broken for over a month when using top-of-tree clang. This also really ought to be fixed before 10.0.

See: bc633a42

Reviewers: chandlerc, mclow.lists, EricWF, ldionne, CaseyCarter

Reviewed By: EricWF

Subscribers: broadwaylamb, hans, dexonsmith, tstellar, llvm-commits, libcxx-commits

Tags: #libc, #llvm

Differential Revision: https://reviews.llvm.org/D72980