MFC r251066:

Fix warnings from newer clang versions about constexpr member functions
not being implicitly const in libc++'s <chrono> header.  The warnings
have been introduced because of new language rules recently adopted by
the C++ WG.  More info:

<http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3598.html>