The stub version of <mstd_type_traits> is mostly identical to the
production file of the same name, except the former lags behind
and doesn't contain the recent "is_constant_evaluated" feature.
It's safe to update the stub file, because "is_constant_evaluated"
only checks generic GCC and Clang versions that also apply to
compilers on PCs.

This enables MbedCRC.h to include <mstd_type_traits> without
distinguishing between Mbed applications and unit tests.