Log Message:
libcxx: updated to 9.0.0

Libc++ 9.0.0

Fixes

Minor fixes to std::chrono operators.
libc++ now correctly handles Objective-C++ ARC qualifiers in std::is_pointer.
std::span general updates and fixes.
Updates to the std::abs implementation.
std::to_chars now adds leading zeros.
Ensure std::tuple is trivially constructible.
std::aligned_union now works in C++03.
Output of nullptr to std::basic_ostream is formatted properly.

Features

Implemented P0608: sane variant converting constructor.
Added ssize function.
Added front and back methods in std::span.
std::is_unbounded_array and std::is_bounded_array added to type traits.
std::atomic now includes many new features and specialization including improved \ 
Freestanding support.
Added std::midpoint and std::lerp math functions.
Added the function std::is_constant_evaluated.
Erase-like algorithms now return size type.
Added contains method to container types.
std::swap is now a constant expression.

Updates

libc++ dropped support for GCC 4.9; we now support GCC 5.1 and above.
libc++ added explicit support for WebAssembly System Interface (WASI).
Progress towards full support of rvalues and variadics in C++03 mode. std::move \ 
and std::forward now both work in C++03 mode.