Fix archetypes.hpp under libcpp-no-extensions and std level < 14

Under -fno-exceptions TEST_THROW becomes abort / __builtin_abort which returns
void. This causes a type mismatch in the conditional operator when testing the
library in C++98,03,11 modes.

Use a comma operator to workaround this problem.

Differential Revision: https://reviews.llvm.org/D26147

llvm-svn: 285572