libstdc++: Make COW string use allocator_traits for nested types

libstdc++: Make COW string use allocator_traits for nested types

When compiled as C++20 the COW std::string fails due to assuming that
the allocator always defines size_type and difference_type. That has
been incorrect since C++11, but we got away with it for specializations
using std::allocator until those members were removed in C++20.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (size_type, difference_type):
	Use allocator_traits to obtain the allocator's size_type and
	difference_type.