libstdc++: Simplify __numeric_traits definition

libstdc++: Simplify __numeric_traits definition

This changes the __numeric_traits primary template to assume its
argument is an integer type. For the three floating point types that are
supported by __numeric_traits_floating an explicit specialization of
__numeric_traits chooses the right base class.

This improves the failure mode for using __numeric_traits with an
unsupported type. Previously it would use __numeric_traits_floating as
the base class, and give somewhat obscure errors for trying to access
the static data members. Now it will use __numeric_traits_integer which
has a static_assert to check for supported types.

As a side effect of this change there is no need to instantiate
__conditional_type to decide which base class to use.

libstdc++-v3/ChangeLog:

	* include/ext/numeric_traits.h (__numeric_traits): Change
	primary template to always derive from __numeric_traits_integer.
	(__numeric_traits<float>, __numeric_traits<double>)
	(__numeric_traits<long double>): Add explicit specializations.