libstdc++: use using instead of typedef for type_traits

Since the type_traits header is a C++11 header file, using can be used instead
of typedef. This patch provides more readability, especially for long type
names.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Use using instead of typedef

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>