[lldb/test] Skip test in TestBitfieldIvars.py instead of xfailing it

The test triggers an ASan exception, causing job failures on the
sanitizer bot.

As suggested by Shafik.

lldb/test/API/lang/objc/bitfield_ivars/TestBitfieldIvars.py

@@ -27,7 +27,9 @@ def test(self):
                                              'field2 =', '3',
                                              'field3 =', '4'])
 
-    @expectedFailureAll()
+    # This test is meant to be xfailed, but running the test triggers an ASan
+    # issue, so it must be skipped for now.
+    @skipIf
     def testExprWholeObject(self):
         self.build()
         lldbutil.run_to_source_breakpoint(self, "// break here", lldb.SBFileSpec("main.m"))