[libc++] Declare noop_coroutine() with _LIBCPP_INLINE_VISIBILITY

[ldionne]

Summary

It was defined with the right visibility, but declared without any visibility.
This function was left out of a prior revision that did the same to several
functions in <compare> (r336665) because the compiler I used didn't support
coroutines. This reinforces the need for automated checks -- there might
still be several cases of this throughout the library.

libcxx/include/experimental/coroutine

public:
   _LIBCPP_CONSTEXPR explicit operator bool() const _NOEXCEPT { return true; }
   _LIBCPP_CONSTEXPR bool done() const _NOEXCEPT { return false; }
 
   _LIBCPP_CONSTEXPR_AFTER_CXX17 void operator()() const _NOEXCEPT {}
   _LIBCPP_CONSTEXPR_AFTER_CXX17 void resume() const _NOEXCEPT {}
   _LIBCPP_CONSTEXPR_AFTER_CXX17 void destroy() const _NOEXCEPT {}
 
 private:
+  _LIBCPP_INLINE_VISIBILITY
   friend coroutine_handle<noop_coroutine_promise> noop_coroutine() _NOEXCEPT;
 
   _LIBCPP_INLINE_VISIBILITY coroutine_handle() _NOEXCEPT {
     this->__handle_ = __builtin_coro_noop();
   }
 };
 
 using noop_coroutine_handle = coroutine_handle<noop_coroutine_promise>;