libstdc++: Remove std::__is_pointer and std::__is_scalar [PR115497]

libstdc++: Remove std::__is_pointer and std::__is_scalar [PR115497]

This removes the std::__is_pointer and std::__is_scalar traits, as they
conflicts with a Clang built-in.

Although Clang has a hack to make the class templates work despite using
reserved names, removing these class templates will allow that hack to
be dropped at some future date.

libstdc++-v3/ChangeLog:

	PR libstdc++/115497
	* include/bits/cpp_type_traits.h (__is_pointer, __is_scalar):
	Remove.
	(__is_arithmetic): Do not use __is_pointer in the primary
	template. Add partial specialization for pointers.