libstdc++: Replace std::__to_address in C++20 branch in <string>

As noted by Patrick, r15-4546-g85e5b80ee2de80 should have changed the
usage of std::__to_address to std::to_address in the C++20-specific
branch that works on types satisfying std::contiguous_iterator.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (assign(Iter, Iter)): Call
	std::to_address instead of __to_address.

Reviewed-by: Patrick Palka <[email protected]>

Author: jwakely

libstdc++-v3/include/bits/basic_string.h

@@ -1748,7 +1748,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	    {
 	      __glibcxx_requires_valid_range(__first, __last);
 	      return _M_replace(size_type(0), size(),
-				std::__to_address(__first), __last - __first);
+				std::to_address(__first), __last - __first);
 	    }
 #endif
 	  else