[libc++] Fix chrono::duration constructor constraint

As per [time.duration.cons]/1, the constructor constraint should be on
const Rep2&. As it is now the code will fail to compile in certain
cases, for example (https://godbolt.org/z/c7fPrcTYM):

     struct S{
          operator int() const&& noexcept = delete;
          operator int() const& noexcept;
     };

     const S &fun();

     auto k = std::chrono::microseconds{fun()};

Differential Revision: https://reviews.llvm.org/D118902