libstdc++: Fix std::regex_replace for strings with embedded null [PR103664]

libstdc++: Fix std::regex_replace for strings with embedded null [PR103664]

The overload of std::regex_replace that takes a std::basic_string as the
fmt argument (for the replacement string) is implemented in terms of the
one taking a const C*, which uses std::char_traits to find the length.
That means it stops at a null character, even though the basic_string
might have additional characters beyond that.

Rather than duplicate the implementation of the const C* one for the
std::basic_string case, this moves that implementation to a new
__regex_replace function which takes a const C* and a length. Then both
the std::basic_string and const C* overloads can call that (with the
latter using char_traits to find the length to pass to the new
function).

libstdc++-v3/ChangeLog:

	PR libstdc++/103664
	* include/bits/regex.h (__regex_replace): Declare.
	(regex_replace): Use it.
	* include/bits/regex.tcc (__regex_replace): Replace regex_replace
	definition with __regex_replace.
	* testsuite/28_regex/algorithms/regex_replace/char/103664.cc: New test.