Add test for spaceship operator to __config
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.
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