Remove XFAIL from test that no longer fails

This test started passing with the old ABI with r263808, so doesn't need
to be marked XFAIL now.

	* testsuite/21_strings/basic_string/requirements/
	explicit_instantiation/debug.cc: Remove XFAIL for old ABI.

From-SVN: r267578

libstdc++-v3/ChangeLog

 2019-01-04  Jonathan Wakely  <jwakely@redhat.com>
 
+	* testsuite/21_strings/basic_string/requirements/
+	explicit_instantiation/debug.cc: Remove XFAIL for old ABI.
+
 	* testsuite/27_io/filesystem/filesystem_error/copy.cc: Fix static
 	assertion failures with old std::string ABI.
 

libstdc++-v3/testsuite/21_strings/basic_string/requirements/explicit_instantiation/debug.cc

@@ -20,7 +20,6 @@
 #include <debug/string>
 
 // { dg-do compile }
-// { dg-xfail-if "COW string missing some required members" { ! cxx11-abi } }
 
 // libstdc++/21770
 namespace debug = __gnu_debug;