Teach the explicit constructor check about constexpr.
Teach the explicit constructor check about constexpr.

If a constructor is marked constexpr it evades the explicit constructor
check right now, since the check only knows about the inline keyword.
Teach it that constexpr can be used also.

google/styleguide,theandrewdavis/cpplint,google/styleguide,google/styleguide,google/styleguide