[libc++] [NFC] s/_LIBCPP_STD_VER > 17 &&...

[libc++] [NFC] s/_LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CONCEPTS)/!defined(_LIBCPP_HAS_NO_CONCEPTS)/

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