libstdc++: One more change for Clang to support constexpr std::string [PR103295]

All writes into the allocated buffer need to be via traits_type::assign
to begin lifetimes.

libstdc++-v3/ChangeLog:

	PR libstdc++/103295
	* include/bits/basic_string.tcc (_M_construct): Use the
	traits assign member to write into allcoated memory.

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

@@ -201,7 +201,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 		_M_data(__another);
 		_M_capacity(__capacity);
 	      }
-	    _M_data()[__len++] = *__beg;
+	    traits_type::assign(_M_data()[__len++], *__beg);
 	    ++__beg;
 	  }