[libc++] Replace _LIBCPP_HAS_NO_CONCEPTS with _LIBCPP_STD_VER > 17. NFCI. All supported compilers that support C++20 now support concepts. So, remove _LIB_LIBCPP_HAS_NO_CONCEPTS in favor of _LIBCPP_STD_VER > 17. Similarly in the tests, remove // UNSUPPORTED: libcpp-no-concepts. Differential Revision: https://reviews.llvm.org/D121528
[libc++] Remove duplicate tests for callable concepts This is essentially a revert of c7ad02009. Indeed, it seems that both 96dbdd75 and c7ad02009 were committed, but c7ad02009 seems to be only an older version of 96dbdd75's tests.