layers: Create instance & device extension lists

Added constant lists of device and instance extension names to
vk_extension_helper.h.

Change-Id: I4273aea866cbfea176a2631e04223ce92443caf0
diff --git a/scripts/helper_file_generator.py b/scripts/helper_file_generator.py
index c588845..114f47f 100644
--- a/scripts/helper_file_generator.py
+++ b/scripts/helper_file_generator.py
@@ -660,6 +660,15 @@
             struct += '    }\n'
             struct += '};\n'
             struct += '\n'
+            # Output reference lists of instance/device extension names
+            struct += 'static const char * const k%sExtensionNames = \n' % type
+            for ext_name, ifdef in extension_dict.items():
+                if ifdef is not None:
+                    struct += '#ifdef %s\n' % ifdef
+                struct += '    %s\n' % ext_name
+                if ifdef is not None:
+                    struct += '#endif\n'
+            struct += ';\n\n'
         extension_helper_header += struct
         extension_helper_header += '\n'
         extension_helper_header += '#endif // VK_EXTENSION_HELPER_H_\n'