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
For example, `std::ranges::range<Holder<Incomplete>*>` should be
well-formed false, not a hard error at compile time.
Differential Revision: https://reviews.llvm.org/D116239