[cxx-interop] Support `char8_t` C++20 type
https://en.cppreference.com/w/cpp/keyword/char8_t
This is based on a patch from Varun Gandhi: https://github.com/swiftlang/swift/pull/26153
rdar://39988329 / resolves https://github.com/swiftlang/swift/issues/68726
