[libcxx] Add missing static_assert error messages

[jasonl220]

Summary

Add error messages to a couple of static_asserts to match the style used in the rest of the file.

Also fix an extra paren typo in a assert error message.

[ldionne]

Do you have commit access?

[jasonl220]

No I don't, can you commit for me please?

Thanks

libcxx/trunk/include/span

// [span.cons], span constructors, copy, assignment, and destructor
     _LIBCPP_INLINE_VISIBILITY constexpr span(      array<value_type, _Extent>& __arr) noexcept : __data{__arr.data()} {}
     _LIBCPP_INLINE_VISIBILITY constexpr span(const array<value_type, _Extent>& __arr) noexcept : __data{__arr.data()} {}
 
     template <class _Container>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span(      _Container& __c,
             enable_if_t<__is_span_compatible_container<_Container, _Tp>::value, nullptr_t> = nullptr)
         : __data{_VSTD::data(__c)}
-        { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (container))"); }
+        { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (container)"); }
 
     template <class _Container>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span(const _Container& __c,
             enable_if_t<__is_span_compatible_container<const _Container, _Tp>::value, nullptr_t> = nullptr)
         : __data{_VSTD::data(__c)}
         { _LIBCPP_ASSERT(_Extent == _VSTD::size(__c), "size mismatch in span's constructor (const container)"); }
 
inline _LIBCPP_INLINE_VISIBILITY
         : __data{__other.data()}, __size{__other.size()} {}
 
 //    ~span() noexcept = default;
 
     template <ptrdiff_t _Count>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> first() const noexcept
     {
-        static_assert(_Count >= 0);
+        static_assert(_Count >= 0, "Count must be >= 0 in span::first()");
         _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
         return {data(), _Count};
     }
 
     template <ptrdiff_t _Count>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> last() const noexcept
     {
-        static_assert(_Count >= 0);
+        static_assert(_Count >= 0, "Count must be >= 0 in span::last()");
         _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
         return {data() + size() - _Count, _Count};
     }
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> first(index_type __count) const noexcept
     {
         _LIBCPP_ASSERT(__count >= 0 && __count <= size(), "Count out of range in span::first(count)");