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:
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>

MFC after:	3 days

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