At the time libcxx does not work as expected with -std=c++17 -fchar8_t.
In my POV, this option was indended to enable the entire [[ http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0482r6.html | P0482 ]], as it has landed into the standard.
At least [[ https://clang.llvm.org/cxx_status.html | Clang C++ Status ]] says so:
//Prior to Clang 8, this feature// (i. e. entire proposal – Yuriy) //is not enabled by -std=c++20, but can be enabled with -fchar8_t. //
This PR makes libcxx to define u8string / u8string_view regarding the standard feature flag, disregarding wrether the standard was switched to C++20.