Per Discord discussion, we're normalizing on a simple `!defined(_LIBCPP_HAS_NO_CONCEPTS)`
so that we can do a big search-and-replace for `!defined(_LIBCPP_HAS_NO_CONCEPTS)`
back into `_LIBCPP_STD_VER > 17` when we're ready to abandon support for concept-syntax-less
compilers.
Differential Revision: https://reviews.llvm.org/D118748
This does not include `std::compare_*_fallback`; those are coming later.
There's still an open question of how to implement std::strong_order
for `long double`, which has 80 value bits and 48 padding bits on x86-64,
and which is presumably *not* IEEE 754-compliant on PPC64 and so on.
So that part is left unimplemented.
Differential Revision: https://reviews.llvm.org/D110738
Arthur O'Dwyer