backport libc++ fixes

[nmm0]

See: https://reviews.llvm.org/D148067#change-zoxBV8KQDQnh

• [ ] We need to use maybe_empty_array because of device code rather than std::array
• [ ] Precondition checks with __is_representable_as
• [ ] Maybe also backport some tests since the LLVM ones are much more extensive