Make IS_TRIVIALLY_CONSTRUCTIBLE consistent on GCC < 5
Make IS_TRIVIALLY_CONSTRUCTIBLE consistent on GCC < 5

std::is_trivially_constructible<T> is equivalent to std::is_trivially_default_constructible<T>
std::has_trivial_default_constructor<T> is the GCC < 5 name for std::is_trivially_default_constructible<T>

std::is_trivial was also used when compiling with clang, due to clang's use of __GNUC__. Test __clang__
to target the intended implementations.

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