Enable TestDataFormatterStdIterator with GCC.

[sivachandra]

Summary

This test now passes for Clang and GCC. I do not know why it was
disabled for GCC with link to a bug which should not have had an
effect on this test.

test/functionalities/data-formatter/data-formatter-stl/libstdcpp/iterator/TestDataFormatterStdIterator.py

class StdIteratorDataFormatterTestCase(TestBase):
     @dsym_test
     def test_with_dsym_and_run_command(self):
         """Test data formatter commands."""
         self.buildDsym()
         self.data_formatter_commands()
 
     @dwarf_test
     @expectedFailureFreeBSD("llvm.org/pr20548") # fails to build on lab.llvm.org buildbot
-    @expectedFailureGcc # llvm.org/pr15301 LLDB prints incorrect sizes of STL containers
     @expectedFailureIcc # llvm.org/pr15301 LLDB prints incorrect sizes of STL containers
     def test_with_dwarf_and_run_command(self):
         """Test data formatter commands."""
         self.buildDwarf()
         self.data_formatter_commands()
 
     def setUp(self):
         # Call super's setUp().