Make IS_TRIVIALLY_CONSTRUCTIBLE consistent on GCC < 5, don't patch clang #14031

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>

https://en.cppreference.com/w/cpp/types/is_default_constructible https://www.gnu.org/software/gcc/gcc-5/changes.html

std::is_trivial was also used when compiling with clang, due to clang's use of __GNUC__. Test __clang__ to target the intended implementations. https://stackoverflow.com/a/28166605

All callers currently only pass one template argument to IS_TRIVIALLY_CONSTRUCTIBLE, with this change the build would fail if someone attempted passing more.

ACK f1640d093fa682c98b000e377916cc32b2267e23 Tested on gcc 4.8.4.