libstdc++: Simplify std::regex_traits::value

libstdc++: Simplify std::regex_traits::value

We don't need to use an istringstream to convert a hex digit to its
numerical value. And if we don't use istringstream there, we don't need
to include <sstream> in <regex>.

libstdc++-v3/ChangeLog:

	* include/bits/regex.tcc (regex_traits::value): Implement
	without using istringstream.
	* include/std/regex: Do not include <sstream>.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>