libstdc++: Fix test that fails in C++20 mode [PR 99995]

The 17_intro/headers/c++1998/49745.cc test fails for C++20 mode with PCH
enabled, because PCH makes it include <bits/stdc++.h>, which includes
<atomic>, and that includes <unistd.h> in C++20 mode. The <unistd.h>
dependency should go away when C++20 atomic waiting is stable, but will
probably remain while the feature is experimental. Change the test to
always include <bits/stdc++.h>, and XFAIL for C++20 and later.

libstdc++-v3/ChangeLog:

	PR libstdc++/99995
	* testsuite/17_intro/headers/c++1998/49745.cc: Include all
	standard headers and XFAIL for effective-target c++20.

libstdc++-v3/testsuite/17_intro/headers/c++1998/49745.cc

@@ -18,5 +18,7 @@
 // <http://www.gnu.org/licenses/>.
 
 // libstdc++/49745
-#include <iostream>
+#include <bits/stdc++.h>
 int truncate = 0;
+
+// { dg-xfail-if "PR libstdc++/99995" { c++20 } }