[libc++] Move a few XFAILs to UNSUPPORTED for consistency

The wide stream tests should be marked as `UNSUPPORTED: no-wide-characters`,
not XFAIL.

Author: ldionne

libcxx/test/std/input.output/iostream.objects/wide.stream.objects/wcerr.sh.cpp

@@ -10,7 +10,7 @@
 
 // wostream wcerr;
 
-// XFAIL: no-wide-characters
+// UNSUPPORTED: no-wide-characters
 
 // RUN: %{build}
 // RUN: %{exec} %t.exe 2> %t.actual

libcxx/test/std/input.output/iostream.objects/wide.stream.objects/wcin.sh.cpp

@@ -17,7 +17,7 @@
 
 // wistream wcin;
 
-// XFAIL: no-wide-characters
+// UNSUPPORTED: no-wide-characters
 
 // RUN: %{build}
 // RUN: echo -n 1234 | %{exec} %t.exe

libcxx/test/std/input.output/iostream.objects/wide.stream.objects/wcout.sh.cpp

@@ -10,7 +10,7 @@
 
 // wostream wcout;
 
-// XFAIL: no-wide-characters
+// UNSUPPORTED: no-wide-characters
 
 // RUN: %{build}
 // RUN: %{exec} %t.exe > %t.actual