Simplify traits.h using C++17

This patch simplifies gdbsupport/traits.h by reusing some C++17 type
traits.  I kept the local names, since they are generally better.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31423
Approved-By: Simon Marchi <simon.marchi@efficios.com>
1 file changed